[11] DEVELOPMENT OF THE OYSTER. 901 



is uo cavity of segmentatiou ; we have here, so to say, an intermediate 

 form between an embolic gastrula and an epibolic gastrula. This latter 

 form seems, moreover, to occur with other marine lamellibranchiata. 

 EabP^ and others have also shown that these two types of the gastrula 

 so different in appearance, are connected together by a series of inter- 

 mediate forms, and may be accounted for by the same process. The 

 embryo of the oyster during this phase presents a remarkable phenom- 

 enon, namely, that there is not only an invagiuation at the vegetative 

 pole, but also a very distinct depression at the other pole, a little below 

 the dorsal side. When seen from the side, this depression of the em- 

 bryo is noticed at once (Fig. 9, sk), and an imaginary line (drawn as 

 shown in Fig. 8) shows at once that it is due to a slight invagination ot 

 the ectodermic cells. During the further development (Figs. 10 and 12) 

 there is formed a small depression, composed of high cylindrical cells, 

 with a narrow opening ; the bottom of this depression is turned toward 

 the dorsal pole of the embryo, while the opening is found across the 

 large axis of the embryo. This little pouch is certainly nothing else 

 but the preconchylian gland, as is shown by observations of more ad- 

 vanced stages. The opinion of Fol,='* that in the oyster the pre- 

 conchylian gland is not an invagiuation properly so-called, but only an 

 ectodermal enlargement, is therefore not correct, and rests probably on 

 observations of more advanced larvse, in which, as in other mollusks 

 with an outer shell, the invagination gradually disappears. This organ 

 was first discovered by Mr. Kay Lankester in the Fisidinm and several 

 gasteropods ; ^^ later Mr. Hatschek discovered it in Teredo.-*^ If one com- 

 pares these two species with Ostrea edulis, it will be found that the pre- 

 conchylian gland appears very early in the embryonic life of the latter. 

 The first naturalists who have studied the history of the development 

 of the oyster, Messrs. Davaine and De Lacaze-Duthiers, speak of an 

 ^^ eohancrure^^ and of a "depression," which gives to the embryo a shape 

 resembling a heart. They seem, therefore, to have known of this in- 

 vagination, although they did not discover its true significance. Ac- 

 cording to the researches of Brooks, the embryo of the Ostrea virginiana 

 shows also in its dorsal part a depression, which, however, according to 

 his statement, contains the opening of the primitive intestinal canal — 

 the blastopore. If one, however, compares Fig. 32 of his work with my 

 Figs. 9, 10, and 12, 1 believe that it must be admitted as highly probable, 

 that what Brooks has taken for the blastoiwre is nothing but the open- 

 ing of the preconchylian gland. This view seems also to be confirmed 

 by what he says of the origin of the shell, which actually begins to de- 

 velop at the very point where the supposed blastopore is found. An 



^ Entwicklung der Tellerschnecke. Morpholog. Jahrbnch, vol. v, p. 601. 

 ^ Etudes sur le ddveloppement des Mollusquea. Arvhiv. de Zoologie Experim., vol. iv, 

 p. 186. 

 "*See his work quoted above, p. 6. 

 «6 Ueher die EnUvicMwngsgeschickte von Teredo. Arb. Zool. Inst., Vienna, vol. iii. 



