EXPLANATION OF PLATE IX & X. 



Fig. 49.— Discogastrula of a Shark (Mustelus). After Balfour (" Develop- 

 ment of the Elasmobranch Fishes," 'Quart. Journ. Microsc. Science,' 

 1874, Vol. xiv, Plate xiii, fig. 1). 



Fig. 50. — Discogastrula of a Boney Fish (Esox). After LerebouUet 

 (' Recherches d'Erabryol. comparee,' 1853, Plate i, fig. 27). 



Fig. 51. — Amphimorula of Bombinator. After Goette ('Entwick. der 

 Unke,' 1875, Taf. ii, fig. 27). 



Fig. 52. — Amphiblastula of Bombinator. After Goette (Ibid., Taf. ii, 

 fig. 28). 



Fig. 53. — Amphigastrula of Bombinator. After Goette (Ibid., Taf. ii, 

 fig. 33). 



Fig. 54. — Discogastrula of the Chick. After Goette ("Die Bildung der 

 Keimblatter im Huhnerei," ' Archiv f. Mikrosk. Anatomic,' Vol, x, 1874, 

 Taf. x, fig. 4). 



PLATE IX. 

 Superficial cleavage and Perigastrula of a Crustacean (Peneus). 



Fig. 81. — Second cleavage-phase : egg with four cleavage- cells (blasto- 

 meres, Huxley) seen from the surface. Four cells have been separated by 

 two annular grooves, an equatorial and a meridianal, passing at right angles 

 to one another, whilst the central food-yelk remains undivided. 



Fig. 82. — The same egg seen in meridianal section. 



Fig. 83. — Fifth cleavage-phase : egg with 32 blastomeres, seen from the 

 surface. 



Fig. 84. — The same egg in meridianal section. 



Fig. 85. — Perimorula (and at the same time Periblastula), seen from the 

 surface. After the completion of the cleavage-process the blastomeres 

 form on the surface of the egg, a single compact layer of equiformal cells 

 (blastoderm), which encloses the internal uncleft food-yelk. 



Fig. 86. — The same Perimorula in meridianal section. 



Fig. 87. — Perigastrula, seen from the surface ; in the middle is the 

 'Urraund' (blastopore), o, which leads into the invaginated 'Urdarm' 

 (archenteron). 



Fig. 88. — The same Perigastrula in section, a, Urdarm (archenteron), 

 and 0, Urmund (blastopore) ; e, exoderm ; i, entoderm ; m, mesoderm. 



Fig. 89. — Nauplius-stage, seen from the ventral surface. I, Upper-lip ; 

 I, II, III, rudiments of the first three pairs of appendages. 



Fig. 90. — The same nauplius-stage in fore-and-aft vertical section 

 (seen from the left side), a, Archenteron ; o, blastopore ; m, mesoderm- 

 cells; /, upper-lip; p, invagination of the pharynx and gizzard (stomo- 

 daeum, L.) ; e, exoderm ; i, entoderm. 



PLATE X. 

 Unequal cleavage and Amphigastrula. 



Figs. 91 — 102. — Unequal cleavage and Amphigastrula of a Chsetopodous 

 Annelid (Fabricia). 



Fig. 91. — Amphimonerula. After fertilization the germinal vesicle has 

 disappeared, and a cytod has arisen from the copulation of sperm-cell and 

 egg-cell. 



Fig. 92. — Amphicytula, the first cleavage-sphere. 



