THE FORMATION OF THE GERM-LAYERS. 



115 



to the gastrula (Fig. 44 B). When gastrulation commences, and 

 during its course, the cleavage-cavity is but slightly developed, or 

 even entirely degenerates, but enlarges considerably at a later 

 period through the greater development of the ectoderm. The 

 archenteric cavity also is large (Fig. 44 (7), and the archenteron thus 

 represents a wide sac (FoL, No. 31). These stages resemble those of 

 Paludina. 



The partial circumcrescence of the macromeres by the ectoderm, 

 as it occurs in the last-named form, is a first indication of the 

 transition to the epibolic gastrula which is formed at an early stage 

 in the Pteropoda (Cymbulia, Clione). The cleavage-cavity here is 

 either entirely reduced or but slightly indicated. The thin layer of 

 ectoderm-cells then lies in close contiguity to the entoderm (Fig. 45 

 A). But even here an invagination takes place. The middle ento- 



ecf.-' 



FIG. 45. .1 and B, embryos of Clione limacina showing the formation of the germ-layers 

 (after KNIPOWITSCH). bl, blastopore ; ect, ectoderm ; ent, entoderm ; mes, meso- 

 derni. 



derm-cells shift upwards, the ectoderm at the same time growing out 

 still further towards the vegetative pole and thus narrowing the 

 blastopore, and the epibolic gastrula thus has the appearance of an 

 invagination-gastrula (Fig. 45 B). A similar process was described 

 in connection with the Lamellibranchia (Ostrea, p. 27). 



The gastrula arises by epibole in Fusus (BOBRETZKY, No. 11), 

 Aplysia (BLOCHMANN, No. 8), Crepidula (CONKLIN, No. 24) and 

 Vermetus (SALENSKY, No. 99). In these forms, the ectoderm, as a 

 thin layer, surrounds the four yolk-laden macromeres, from which, 

 at a later stage, small cells become detached, chiefly at the vegetative 

 pole, that is, in the neighbourhood of the blastopore ; by the develop- 

 ment of these small cells an archenteron is formed, bounded dorsally 

 by the four macromeres and ventrally by these small cells. In Neri' 

 Una, these cells form early, before the circumcrescence of the macro- 



