BULLETIN OF THE UNITED STATES FISH COMMISSION. 1G1 



inated, and that in this way a true gastrula stage has been developed. 

 It is plain, however, that no true gastrulation takes place, since it is 

 not possible to demonstrate a true cleavage cavity; indeed it appears 

 to be, as it were, a transition form between a gastrula formed by em- 

 bolic invagination and one developed by the epibolic downgrowth of 

 the ectoderm over the entoderm. This last form appears to be common 

 to other marine lamellibranchs. Indeed Eabl * has already pointed out 

 that these apparently and fundamentally different modes of the forma- 

 tion of the gastrula are connected together by a series of transition 

 forms, and that both may be referred to essentially the same process. 



The embryo oyster at this stage is remarkable in that there is not 

 only an invagination at the vegetative pole, but that there is also visi- 

 ble a distinct transverse groove formed a little below the apex of the 

 opposite pole. When the. embryo is viewed from the side the latter in- 

 vagination immediately becomes apparent (Fig. 5, si-), and an optic sec- 

 tion (Fig. 4) teaches us that it has originated from a mass of ectoderm 

 cells which have been pushed inwards towards the center of the embryo. 

 In the course of further development (Figs. 7 and 8), a sack with a nar- 

 row cavity is developed from this invagination, the walls of which are 

 formed of long, cylindrical cells; the blind end of the sack is now di- 

 rected towards the dorsal side of the embryo, whilst the direction of its 

 cavity is parallel with the longitudinal axis of the latter. Without 

 doubt, as we see in the case of older stages, this sack or invagination 

 is nothing more than the shell gland. The assertion of Folf that the 

 shell gland in the embryos of Ostrea is not a true invagination, but that 

 it is merely an ectodermal thickening, slightly hollowed out, is thus 

 seen not to be very just, and apparently rests on what is observed in 

 the older stages, where, as in other embryo mollusks provided with an 

 external shell, the invagination becomes gradually shallower. As is 

 well known, this organ was first observed in the Cephalophora, and was 

 afterwards met with by Ray Lankesterf and Hatschek§ amongst the 

 lamellibranchs (Pisklium and Teredo) ; in comparison with the genera 

 just named, the shell gland of the oyster appears very early in embry- 

 onic life. 



The first investigators who studied the development of the oyster, 

 Davaine and Lacaze-Duthiers, speak of " une echancrure" and " une 

 depression" from the presence of which the embryo becomes heart- 

 shaped when viewed from the side ; this invagination therefore appears 

 to have been known to the older authors, although its significance was 

 not understood by them. According to the investigations of Brooks, the 

 embryo of Ostrea virginiana also has a deep depression or groove on 



* Entwickelung der Tellerschnecke. Morph. Jabrbueh, Bd. V, p. 601. 



t Etudes sur le de>eloppement des Mollusques. Arcli. de Zoologie ixpe'r., T. vi, p. 

 186. 



tOu the developmental history of Mollusca. Philos. Transac. Eoy. Soc, 1874. 



9 Ueber Entwickelungsgesckichte vou Teredo. Arbeiteu aus dein Zool. lust. Wien.. 

 T. III. 



Bull. U. S. F. C, 82 11 Feb, 1, 1883. 



