Text-book of the embryology of invertebrates (Volume 4) online

Bd. iv. 1889. Suppl.
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Zool. Suisse. Torn. i. 1884 ; also " Remarques," etc., op. cit.
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Ascidians. Proc. Roy. Soc. London. Vol. li. 1'892.
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a la parente des Vertebres et des Tuuiciers. Archiv. ZooL

r^er. Tom. i. 1872.

23. GIARD, A. Sur I'embryogenie des Ascidies du genre Lithon-

ephria. Compt. Rend. Acad. Sci. Paris. Tom. xcii. 1881.

24. HERDMAN, W. A. Report on the Tunicata. Challenger

Reports. Part I. Vol. vi. 1882. Part II. Vol. xiv. 1886.
Part III. Vol. xxvii. 1888.

25. HERTWIG, O. Untersuchungen iiber den Ban und die Entwick-

lung des Cellulosemantels der Tunicaten. Jen. Zeitschr. f.
Nat (mi-. Bd. vii. 1873.



526 TUNICATA.

36. JULIN, CH. Recherches sur 1 'organisation des Ascidies simples.

Arcltir. Bio/. Tom. ii. 1881.
27. JULIN C. Recherches sur 1'organisation des Ascidies simples.

ii. Sur 1'hypophyse et quelques organes qui s'y rattachent

chez Ascidia compressa et Phallusia mammillata. Arehir.

BioL Tom. ii. 1881.
"28. KINGSLEY, J. S. Some points in the development of Molgula

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29. KOWALEVSKY, A. Entwicklungsgeschichte der einfachen Asci

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30. KOWALEVSKY, A. Weitere Studien iiber die Entwicklung der

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31. KOWALEVSKY, A. Ein Beitrag zur Kemitniss der Excretions-

organe. BioL Centralbl. Bd. ix. 1889.

32. KOWALEVSKY, A. Einige Beitrage zur Bildung des Mantels der

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33. KROHN, A. Ueber die Entwicklung der Ascidien. Mnller'a

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34. KUPFFER, C. Die Stammverwandtschaft zwischen Ascidien

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35. KUPFFER, C. Zur Entwicklung der einfachen Ascidien'

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36. LACAZE-DUTHIERS, H. BE. Les Ascidies simples des cotes de

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37. LAHILLE, F. Sur le developpement typique du systeme nerveux

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38. LAHILLE, F. Recherches sur les Tuniciers des cotes de France.

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39. MAURICE, CH. ET SCHULGIN. Embryogenie de 1'Amaroecium

proliferum (Ascidie composee). Ann. Sci. Nat. (6). Tom.
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40. MAURICE, CH. Etude monographique d'une espece d'Ascidie

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41. METSCHNIKOFF, E. Observations sur le developpement de

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LITERATURE. 527

42. METSCHNIKOFF, E. Zur Entwicklung der einfachen Ascidien.

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43. METSCHNIKOFF, E. Vergl. embryologische Studien. (3) Ueber

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44. METSCHNIKOFF, E. Utersuchungen iiber die intracellulare

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45. MILNE-EDWARDS, H. Observations sur les Ascidies composees

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46. MORGAN, T. H. The Origin of the Test-cells of Ascidians.

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47. EOULE, L. Sur le developpement des enveloppes ovulaires chez

les Tuniciers. Bee. Zool. Suisse. Tom. ii. 1885.

48. SABATIER, AD. Recherches sur 1'ceuf des Ascidiens. Revue

/. AV. Mnntitellier. Tom. xi. 1883.

49. SALENSKY, W. Ueber die Thatigkeit der Kalymmocyten bei

der Entwicklung der Ascidien. Festschrift fur Leuckart.
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50. SEELIGER, Osw. Die Entwicklungsgeschichte der socialen

Ascidien. Jen. Zeitschr. f. Naturw. Bd. xviii. 1885.

51. SEMPER, C. Ueber die Entstehung der geschichteten Cellulose-

epidermis der Ascidien. Arb. Zool . Instit at Wurzburg. Bd. ii.
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52. SHELDON, L. Note on the Ciliated Pit of Ascidians and its

Relations to the Nerve-ganglion and the so-called Hypophysial
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53. Ussow, M. Zoologisch - embryologische Untersuchungen.

Archiv. fur Nalurgesch. Jahrg. xli. 1875.

54. WILLEY, A. On the Development of the Hypophysis in the

Ascidians. Zool. An?.. Jahrg. xv. 1892.

54r*. WILLEY, A. Studies on the Protochordata. Quart. Journ.
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Asexual Reproduction of the Ascidians.

55. GANIN, M. Neue Thatsachen aus der Entwicklungsgeschichte

der Ascidien. Zeitschr. f. ivisx. Zool. Bd. xx. 1870.

56. GANIN, M. Notes on the Development of Didemnum and

Botryllus. Protjr. Univ. Warschau. 1870. (Russian.)



528 TUNICATA.

57. GIARD, A. Reeherches sur les Synaseidies. Arrhiv. ZoL

exper. Vol. i. 1872.

58. GIARD, ALFR. Sur le bourgeonnement des larve,s d'Astellium

spongiforme Ud. et sur la Poecilogenie chez les Ascidiets.
Compt. Rend. Acad. Sci. Paris. Tom. cxii. 1891.

59. HJORT, J. Zum Entwicklimgscyclus der zusammengesetzten

Ascidien. Zool. Anz. Jahrg xvi. 1892.

60. KOWALEVSKY, A. Sur le bourgeonnement du Perophora Listeri.

Revue. Sci: Nat. Montpellier. 1874.

61. KOWALEVSKY, A. Ueber die Knospung der Ascidien. Archiv.

f. mikro. Anat. Bd. x. 1874.

62. KROHN, A. Ueber die Fortpflanzungsverhaltnisee der Botryl-

liden. Archiv. f. Naturg. Bd. xxxv. 1869.

63. KROHN, A. Ueber die friibeste Bildung der Botryllenstocke.

Archiv. f. Naturgesch. Bd. xxxv. 1869.

64. JOURDAIN, Pi Sur les Ascidies composees de la tribu des

Diplosomidae. Compt. Rend. Acad. Sci. Paris. Tom. c. 1885.
64a. OKA, A. Ueber die Knospung der Botrylliden. Zeitschr. f.

wiss. Zool. Bd. liv. 1892.
646. OKA, A. Die periodische Regeneration der oberen Korperhalfte

bei den Diplosomiden. Biul. Centra! hi. Bd. xii. 1892.

65. PIZON, A. Sur la blastogenese chez les Botryllides. Bull. Sor.

Philom. Paris. (8). Tom. iii. 1891-92.

66. SEELIGER, O. Zur Entwicklung der Ascidien. Eibildung und

Knospung von Clavellina lepadiformis. Sitzuwjisber. Acad.
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67. ULJANIN, B. .Bemerkungen uber die Synascidiengattung

Distaplia. Zool. Anz. 1885.

68. DELLA VALLE, K. Nuove contribuzioni alia storia naturale

delle Ascidie composte del golfo di Napoli. Mem. R. Accad.
Lincei(3). Vol. x. 1881.

69. DELLA VALLE, A. Sur le bourgeonnement des Didemnides et

Botryllides et sur le type enterocoelien des Ascidies. Archir.
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70. DELLA VALLE, A. Sul ringiovanimento delle colonie de

Diazona violacea. Sav. Cum. prelim. Archiv. Ital. Biol.
Vol. v. 1884.

Pyrosoma.

71. KOWALEVSKY, A. Ueber die Entwicklungsgeschichte der

Pyrosoma. Archiv. f. mikro. Anat. Bd. xi. 1875.



LITERATURE. 529

72. HUXLEY, TH. H. Anatomy and Development of Pyrosoma.

Trans. Linn. Soc. London. Vol. xxiii. 1860.

73. JOLIET, L. Etudes anatomiques et embryoge"niques sur le

Pyrosoma giganteum, etc. Paris, 1888.

74. SALENSKY, W. Beitriige zur Entwicklungsgeschichte der

Pyrosomen. Zool. Jahrh. Abtlt. f. Anat. Bd. iv. 1891.
Bd. v. 1892.

7">. SEELIGER, O. Bemerkungen zu Herrn Prof. Salensky's
" Beitrage 7Air Kmbryonalentwicklung der Pyrosomen." Zool.
Am. Jahrg. xv. 1892.

76. SEELIGER, O. Zur Entwicklungsgeschichte der Pyrosomen.

Jr/ia. Zt'.itxi'hr.f. Xatunr. Bd. xxiii. 1889.

76a. SEELIGER, O. Ueber. die erste Bildung des Zwitterapparates
in (den jungen Pyrosomenstocken. Festschrift fur Leuckart.
Leipzig, 1892.

Doliolum.

77. BARROIS, J. Recherches sur le cycle genetique et le bourgeon-

nement de 1'Anchinie. Journ. Anat. Phys. Paris. Ann. xxi.
1885.

78. GEGENBAUR, C. Ueber den Entwicklungscyclus von Doliolum

nebst Bemerkungen liber die Larven dieser Thiere. Zeitschr.
f. iviss. Zool. Bd. vii. 1856.

79. GROBBEN, C. Doliolum und sein Generationswechsel. Arb.

Zool. Inst. Wien. Bd. iv. 1882.

80. HUXLEY, TH. H. Remarks upon Appendicularia and Doliolum.

Phil. Tran*. London. 1851.

81. KEFERSTEIN UND EHLERS. Zoologische Beitrage. Leipzig,

1861.

82. KOROTNEFF, AL. DE. La Dolchinia mirabilis. Mitth. Zool.

Stat. Neapel. Bd. x. 1891.

83. KOROTNEFF, A. Die Knospung der Anchinia. Zeitschr. f.

wins. Zool. Bd. Ix. 1884.

84. KOWALEVSKY, A., ET BARROIS, J. Materiaux pour servir a

1'histoire de 1'Anchinie. Journ. Anat. Phys. Pans. Tom.
xix. 1883.

85. KROHN, A. Ueber die Gattuug Doliolum und ihre Arten.

Archiv.f. Natury. Bd. xviii. 1852.

86. ULJANIN, B. Die Arten der Gattung Doliolum im Golfe von

Neapel. Fauna nn<l Flora van Neapel. Monogr x. 1884.

MM



530 TUNICATA.

Salpa.

87. BARROIS, J. Memoire sur les membranes embryonnaires des

Salpes. Journ. Anat. Phys. Paris. Ann. xvii. 1881.

88. BROOKS, W. K. On the Development of Salpa. Bull. Mus.

Comp. Anat. Harv. Coll. Camb. Vol. iii. 1871-1876. Cf.
Archiv. fur Naturg. Bd. xlii. 1876.

89. BROOKS, W. K. The Origin of the Eggs of Salpa. Stud. Biol.

Lab. Johns Hopkins Univ. Baltimore. Vol. ii. 1882.

90. BROOKS, W. K. Chamisso and the Discovery of Alternation of

Generations. Zool. Anz. Jahrg. v. 1882.

91. BROOKS, W. K. Is Salpa an Example of Alternation of Genera-

tions ? Nature. Vol. xxx. 1884.

92. BROOKS, W. K. The Anatomy and Development of the Salpa-

chain. Stud. Biol. Lab. Johns Hopkins Univ. Baltimore.
Vol. iii. :j&86.

93. BROOKS, W. K. On the Relationship between Salpa and

Pyrosoma. Johns Hopkins Univ. Circ. Vol. 9. 1890.

94. BuTSOHLij O. Einige Bemerkungen iiber die Augen derSalpen.

Zool. Anz. Jahrg. xv. 1892.

94a. GOPPERT, E. Untersuchungen iiber das Sehorgan der Salpen.
Morph. Jahrb. Bd. xix. 1892.

95. HUXLEY, TH. H. Observations on the Anatomy and Physiology

of Salpa and Pyrosoma. Phil. Trans. 1851.

96. KOWALEVSKY, A. Beitrag zur Entwicklung der Timicaten.

Nachr. kgl. Geselhch. Wissensch. Gottingen. 1868.

97. KROHN, A. Observations sur la generation et le developpement

des Biphores. Ann. Sci. Nat. (3). Tom. vi. 1846.

98. LEUCKART, E. Salpa und Verwandte. Zool. Unter&uchnngen,

Heft. ii. Giessen, 1854.

99. METCALF, M. The Anatomy and Development of the Eyes

and Subneural Gland in Salpa. Johns Hopkins Univ. Girc.
No. 97. 1892.

99a. METCALF, M. On the Eyes, Subneural Gland and Central
Nervous System in Salpa. Zool. Anz. Jahrg. xvi. 1893.

100. SALENSKY, W. Ueber die embryonale Entwicklungsgeschichte

derSalpen. Zeitschr. f. wits. Zool. Bd. xxvii. 1876.

101. SALENSKY, W. Ueber die Knospung der Salpen. Morph Jahrb.

Bd.iii. 1877.

102. SALENSKY, W. Ueber die Entwicklung der Hoden und iiber

den Generationswechsel der Salpen. Zetiachr. f. wi**. Zool.
Bd. xxx. Suppl. 1878.



LITERATURE. 531

103. SALENSKY, W. Folliculare Knospuug der Salpen und die

Poly embryonic der Pflanzen. Biol. Gentralbl. Bd. v. 1885.

104. SALENSKY, W. Neue Untersuchungen iiber die embryonal e

Entwicklung der Salpen. Mitth. Zool. Stat. Neapel. Bd. iv.
1883. In two parts.

105. SEELIGER, Osw. Die Knospung der Salpen. Jen. Zeitschr.

fur Naturw. Bd. xix. 1886.

106. SEELIGER Osw. Die Entstehung des Generationswechsels der

Salpen. Jena. Zeitschr. f. Naturw. Bd. xxii. 1888.

107. TODARO, FR. Sopra lo sviluppo e 1'anatomia delle Salpe. Atti.

R. Accad. Lincei. Trans. (2). Vol. ii. 1875.

108. TODARO, FR. Sui primi fenomeni dello sviluppo delle Salpe.

Atti. R. Accad. Lincei. Tram. (3). Vol. iv. 1880.

109. TODARO, FR. Sui primi fenomeni dello sviluppo delle Salpe.

2 da communic. preliminare. Atti. R. Accad. Lincei. Trans.
Vol. vi. 1882. (Also in Archiv. Ital. Biol. Tom. ii. 1882.)

110. TODARO, FR. Sui primi fenomeni nello sviluppo delle Salpe.

3 a comm. prelim. Atti. R. Accad. Lincei. Trans. Vol. vii.
1883. (Also in Archiv. Ital. Biol. Tom. iii. 1883.)

111. TODARO, FR. Sopra i canali e le fessure branchiali delle Salpe.

Atti. R. Accad. Lincei. Trans. Vol. viii. 1884.

112. TODARO, FR. Studi ulteriori sullo sviluppo delle Salpe. Atti.

R. Accad. Lincei. Mem. (4). Vol. i. 1886.

113. TODARO, FR. Sull' omologia della branchia delle Salpe con

quella degli altri Tunicati. Atti. R. Accad. Lincei. Rend. (4).
Vol. vi. 1889. (Also in Archiv. Ital. Biol. Tom. xi. 1889).

APPENDIX TO LITERATURE ON TUNIC AT A.

I. BROOKS, W. K. The genus Salpa. Mem. Johns Hopkins

Univ. 1893.

II. CASTLE, W. E. Early Embryology of Ciona intestinalis.
Kn.ll. Mn*. Haroard. Vol. xxvii. 1896.

III. CASTLE, W. E. On the Cell-lineage of the Ascidian egg.

Pmc. Atmr.Acad. Vol. xxx. 1894.

IV. CAULERY, M. Contributions a T^tude des Ascidies com-

posees. Bull. Sci. France et Bdgiqw. Tom. xxvii.
1895.

V. CAULERY, M. Sur la Morphologic de la Larve composee
d'une Synascidie (Diplosomoides Lacazii, Giard). Compt.
Rend. Acad. Sci. Par IK. Tom. cxxv. 1897.



532 TUNICATA.

VI. CAULEBY, M. Sur le Bourgeonnement des Diplosomidae
et des Didemnidae. Compt. Rend. Acad. Sci. Paris.
Tom. cxxi. 1895.

VII. CAULEBY, M. Sur 1' Interpretation morphologique de la
larve, etc., du genre Diplosoma. Compt. Rend. Acad.
Sci. Paris. Tom. cxxi. 1895.

VIII. CAULEBY, M. Sur les Synascidies du genre Colella, et le
polymorphisme de leurs bourgeons. Compt. Rend. Acad.
Sci. Paris. Tom. cxxii. 1896.
IX. DAMAS, D. Les Formations epicardiques chez Ciona

intestinalis. Arcliiv. Biol. Tom. xvi. 1899.
X. FLODEBUS, M. Uber die Bildung der Follikelhiillen bei

den Ascidien. Zeitschr. f. wiss. Zool. Bd. Ixi. 1896.
XI. GABS^ANG, W. Budding in Tunicata. Science Progress.

VolJiii. 1896.

XII. GIABD, A., ET CAULEBY, M. Sur 1'Hivernage de la
Clavelina lepadiformis. Compt. Rend. Acad. Sci. Paris.
Tom. cxxiii. 1896.

XIII. HEIDEB, K. Beitrage zur Embryologie von Salpa fusi-

formis. Frankfurt, 1895. (Abh. Senkenb. Ges. Bd.
xviii.)

XIV. HJOBT, J. Beitrage zur Keimblatterlehre und Entwick-

lungsmekanik der Ascidienknospung. Anat. Anz. Bd. x.
1895.

XV. HJOBT, J. Ueber den Entwicklungcyclus der zusamcnen-
gesetzten Ascidien. Mittheil. Zool. Stat. Neapel. Bd. x.
1893.
XVI. HJOBT, J., UND BONNEVIE, FB. Ueber die Knospung von

Distaplia magnilarva. Anat. Anz. Bd. x. 1895.
XVII. JULIN, C. Recherches sur la blastogenese chez Distaplia

magnilarva et D. rosea. Congr. Zool. Leyden. 1896.
XVIII. KOBOTNEFF, A. Embryonale Entwicklung der Salpa demo-
cratica. Biol. Centralbl. Bd. xiv. 1894 ; and Zeitschr.
f. wiss. Zool. Bd. lix. 1895.
XIX. KOBOTNEFF, A. Tunicatenstudien. Mitth. Zool. Stat.

Neapel. Bd. xi. 1895.

XX. KOBOTNEFF, A. Zur Embryologie von Salpa cordiformis-
zonaria und musculosa-punctata. Mitth. Zool. Stat.
Neapel. Bd. xii. 1896.

XXa. KOBOTNEFF, A. Zur Embryologie von Salpa runcinata-
fusiformis. Zeitschr f. wiss. Zool. Bd. Ixii. 1896.



LITEBATUBE.



533



XXI. KOKOTNEFF, A. Zur Entwicklung der Salpen. BivL

Gentratbl. Bd. xv. 1895.
XX la. KOBOTNEFF, A. Zur Ernbryologie von Salpa maxima-

Africana. Zeitschr. f. iviss. Zool. Bd. Ixvi. 1899.
XXII. LEFEVBE, G. Budding in Ecteinascidia. Anat. Anz. Bd.

xiii. ; and Jolim Hopkins Univ. Uirc. 1897.

XXIII. LEFEVBE, G. On budding in Perophora. Johns Hopkins

Unic. Circ. Vol. xiv. 1895.

XXIV. METCALF, M. The follicle-cells of Salpa. Zool. Anz.

Jahrg. xx. 1897 ; and Johns Hopkins Univ. Circ.
1897.

XXV. PIZON, A. Contributions a FEmbryogenie des Ascidies
simples. Compt. Rend. Acad. Set. Paris. Tom. cxx.
1895.

XXVI. PIZON, A. Histoire de la Blastogenese chez les Botryllides.

Ann. Sri. Nat. Tom. xiv. 1893.

XXVIa. PIZON, A. Evolution des elements sexuels chez les
Ascidiens composes. Compt. Rend. Acad. Sci. Paris.
Tom. cxix. 1894.

XXVII. PIZON, A. Les Membranes embryonnaires et les Cellules

de rebut chez les Molgules. Compt. Rend. Acad. Sci.

Paris. Tom. cxxii. 1896.
XXVIII. EITTEB, W. E. Budding in Compound Ascidians based

on studies on Goodsira and Perophora. Journ. Morphol.

Vol. xii. 1896.
XXIX. SALENSKY, W. Beitrage zur Entwicklungsgeschichte d.

Synascidien (Diplosoma and Didemnum). Mitth. Zool.

Stat. Neapd. Bd. xi. 1895.
XXX. SALENSKY, W. Morphologische Studien an Tunicaten.

(Xervous system and Metamorphosis of Distaplia). Morph.

Jalirb. Bd. xx. 1893.
XXXI. SALENSKY, W. Ueber die Entstehung der Metagenesis bei

Tunicaten. Bi.ol. CentraWl. Bd. xiii. 1893.
XXXII. SAMASSA, P. Zur. Kenntniss der Furchung bei den Asci-

dien. Archiv. f. mikro. Anat. Bd. xliv. 1894.

XXXIII. SEELIGEB, O. Einige Beobachtungen iiber die Bildung

der auesseren Mantels der Tunicaten. Zeitschr. f. wis*.
Zool. Bd. Ivi. 1893.

XXXIV. SEELIGEB, O. Ueber die Entstehung des Peribranchial-

raums in den Embryonen der Ascidien. Tom. cit.



534 TUNICATA.

XXXV. TODABO, F. Sopra lo Sviluppo della parte anteriore del

corpo delle Salpe. Atti. R. Acad. Lincei. Rend. (3).
Vol. vi. 1897..

XXXVI. WILLBY, A. Studies on the Protochordata. Parts II. and

III. (nervous system, sense-organs and mouth). Quart.
Journ. Micro. Sci. Vol. xxxv. 1893.

XXXVII. WILLEY, A. Amphioxus and the Ancestors of the verte-

brates. London and New York. 1894.



CHAPTER XXXVI.

CEPHALOCHORDA.

Amphioxus.

THE earlier statements concerning the development of Amphioxus
made by MAX SCHULTZE (No. 18), LEUCKART and PAGENSTECHER
(No. 15) referred merely to a few of the later larval stages ; our
knowledge of the ontogeny of this form is, therefore, founded princi-
pally on the investigations of KOWALEVSKY (Nos. 10 and 11), and was
extended by HATSCHEK (Nos. 4 and 8). The metamorphosis of
Amphioxus has recently been described by RAY LANKESTER and
WILLEY (No. 13) and by WILLEY alone (No. 23). The develop-
ment of the genital organs has been investigated by BOVERI (No. 3).
This last author (No. 2) as well as SPENGEL (No. 19), RAY LANKESTER
(No. 12) and VAN WIJHE have also published treatises on the anatomy
of the adult Amphioxus to which we shall have occasion to refer.*

A. Oviposition, Cleavage and Gastrulation.

The mature genital products of Amphioxus pass from the genital
chambers, through rupture of their walls, into the atrial cavity and
thence they were said to pass through the gill-clefts into the pharynx
and to be ejected through the mouth (KOWALEVSKY, HATSCHEK).
According to RAY LANKESTER and WILLEY, however, they are, in
most cases, ejected through the atriopore. Fertilisation takes place
in the surrounding water. The newly laid egg is surrounded by a
vitelline membrane at first only slightly separated from it, but, under
the influence of the sea water, the interval between the egg and the

* [More recently, SOBOTTA (No. XI.) and STICHT (No. XII.) have rein-
vestigated the maturation and fertilisation of the egg, and the former,
KLAATSCH (No. IV.) and MACBRIDK (No. VIII.) have re-examined the forma-
tion of the germ-layer. ED.]



536 CEPHALOCHORDA.

membrane becomes greater. There is no micropyle. The spermatozoa
pass through this elastic membrane to reach the egg.*

The first stages of development closely resemble those of the
Ascidians. Cleavage is total and almost equal (adequal type of
HATSCHEK). The first furrow is meridional and appears first at
the animal pole, where for some time it is deepest ; it eventually
divides the egg into two exactly equal parts (Fig. 279 B). The
second furrow, which is also meridional, is at right angles to the first
and leads to the rise of four blastomeres of equal size which leave free
between them a cavity open above and below ; this is the cleavage-
cavity (Fig. 279 C and Z>). The eight-celled stage (Fig. 279 E) is
brought about by an equatorial furrow which lies somewhat nearer
the animal than the vegetative pole and leads to the first differentia-
tion between the blastomeres of the animal and vegetative halves.
The embryo consists of a circle of four smaller blastomeres near the
animal pole and another circle of four larger blastomeres belonging
to the vegetative pole. Further meridional furrows divide these eight
cleavage -spheres into sixteen, the sixteen-celled stage then consisting
of a circle of eight smaller and another of eight larger blastomeres
(Fig. 279 F).

Even at this sixteen-celled stage, according to WILSON, certain individual
differentiations are found which influence the further course of cleavage. The
regular stage described by HATSCHEK, in which the eight cells of the upper
circle are found resting regularly on the eight cells of the lower circle, was
comparatively rarely observed by WILSON. The blastomeres of the upper
circle often appear shifted spirally in relation to those of the lower, as is found
to be the case in the Annelida and Mollusca. Most frequently, however, at
the sixteen-celled stage a bilateral (or strictly speaking a biradial) symmetry
is evident in the arrangement of the blastomeres, the eight cells of the vege-
tative half being divided into four larger and four smaller cells. The four
larger cells surround the vegetative pole in regular order, while the four
smaller cells are grouped in two pairs symmetrically to the median plane.
This median plane, according to WILSON, corresponds to the first cleavage-
plane. A similar arrangement of the blastomeres was seen in the sixteen-celled
stage of the Ascidians.

The thirty-two-celled stage arises, according to HATSCHEK, in
consequence of further equatorial furrows. It consists of four rows
of eight cells each, one super-imposed above the other. The dilated
cleavage-cavity, which was hitherto open at the animal and vegetative
poles, now becomes closed at these points. In the further course of

[* Only one polar body is generally said to be present at this stage (Fig.
279), but SOBOTTA (No. XI.) has recently discovered the presence of a second
one. ED.]



CLEAVAGE AND GASTRULATION.



537



C




I)



B




Fi;. 'J7H. CltMvuge <>f J /////// /",<-/'.v (after SALENSKY). .4, egg before cleavage, with
the polar body ; tt, division into two, the two cells being still connected by a baud
.if protoplasm; O, four-celled stage; U, the same seen from the pole; E, eight-
celled stage ; F, sixteeu-celled stage ; V, stage showing more rapid division at the
animal pole : one of the circles of cells is in the act of dividing ; //, the same stage
in section ; /, blastula, surface view/ K, blastula, in section.



538



CEPHALOCHORDA.



cleavage, the circle of small blastomeres divides more rapidly, while
the circle of eight larger cleavage-spheres surrounding the vegetative
pole remains longer undivided (Fig. 279 G). In later stages, the
regular arrangement of the blastomeres in circles is obliterated and
the cells form an epithelium surrounding the cleavage-cavity, the
blastula-staye being reached in this way (Fig. 279 /, A'). The egg,
at this stage, is lengthened in the direction of the future gastrula-
axis, and the wall at the vegetative pole, /.<., the posterior third of
the egg, is composed of somewhat larger cells richer in yolk-granules.
This represents the entodermal region of the embryo. In it flatten-
ing occurs, and this soon passes into an invagination (Fig. 280 A)
which leads to the development of a cap-shaped gastrula. The
invagination causes the cleavage-cavity to decrease in size and finally
completely to disappear, the two primary germ-layers coming into
close contact (Fig. 280 B).

C




FIG. 280. Three consecutive ontogenetic stages of Amphioxus (after HATSCHEK),
showing the invagination of the entoderm. A, during invagination ; R, after the
completion of invagination, right dorsal side, left ventral side ; C, with narrowed
gastrula-mouth, orientation as in R.

The gas trul a -stage now passes through certain phases by means of
which the bilateral symmetry which, according to WILSON, is already
evident in the stages of cleavage, becomes more distinct, while at the
same time, the embryo elongates in the direction of its definitive
longitudinal axis. The apex of the ectoderm of the gastrula corre-
sponds to the animal pole, while the vegetative pole may be said to
coincide with the centre of the at first circular aperture of invagina-
tion. This latter soon becomes oval, and the plane of symmetry is
thus established. These later stages, seen in profile (Fig. 280 B\
show a point at which the curve is more abrupt ; this point does not
coincide with the animal pole but lies somewhat excentrically, corre-
sponding to the anterior end of the later principal axis, the posterior



DEVELOPMENT OF THE MEDULLARY TUBE, ETC. 539

end coinciding with the posterior edge of the blastopore. The
definitive principal axis thus forms an acute angle with the primary
axis. The blastopore has undergone shifting to the dorsal side of
the embryo ; it now gradually decreases in size, chiefly through a
backward growth of its dorsal or anterior border. The posterior
(ventral) edge of the blastopore, on the contrary, remains stationary