344 DR. R. HORST. 



Lankester^ and Hatschek^ in some Lamellibranchiata {Pisi- 

 dium, Cyclas, and Teredo). In comparison with these 

 genera the shell-gland of the oyster appears at a very early 

 period. 



The first investigators of the development of the oyster — 

 Davaine and Lacaze-Duthier — have mentioned already " une 

 echancrure''^ and '' une depression," which gives the embryo 

 a cordiform shape ; the invagination of the shell-gland seems, 

 therefore, not to have been unknown to them, but as they 

 studied the embryo only on the surface they have not under- 

 stood the true signification of that pit. According to Brooks' 

 observations, the embryo of Ostrea mrginiana is provided 

 also with a deep semilunar groove, which he nevertheless 

 believes to be the blastopore. Comparing, however, his fig. 

 32 (op. cit.) with mine (figs. 5, 6, and 8), I think it very 

 probable that what Brooks supposed to be the blastopore is 

 nothing but the opening of the shell-gland. This interpre- 

 tation quite agrees with his observation that on the point, 

 where is situated his blastopore, he sees afterwards develop 

 the first traces of the shell. A similar origin of the shell of La- 

 mellibranchiata has till now only been found by RabF in the 

 Unio, and is so contrary to the observations on the develop- 

 ment of other Mollusca that it certainly deserves to be ex- 

 amined again. In a following stage, figured in fig. 6, we 

 see the invagination of the hypoblast completed, forming the 

 archenteron. Behind the blastopore lie a couple of large 

 cells, probably the first mesoblastic cells, whose origin un- 

 happily remained unknown to me. Meanwhile the ventral 

 side of the embryo begins to project a little, forming a kind 

 of foot, which makes the embryo strongly resemble a young 

 Gasteropod. The blastopore is still clearly visible, and 

 shows a somewhat triangular form. As far as I could see it 

 does not disappear during the further development, and 

 directly passes into the permanent mouth, or rather into the 

 cardia. For just as in those embryos whose blastopore 

 remains not open, and whose mouth and oesophagus are 

 formed by an invagination of the epiblast, here also the epi- 

 blastic cells participate in the formation of the oral region of 

 the alimentary tract. 



During the further growth of the embryo great internal 

 as well as external changes take place. The shell-gland 



' "On the Developmental History of Mollusca," 'Phil. Trans, of the 

 Koyal Society,' 1S75. 



2 ' Ueber Entwickelungsgeschichte von Teredo.' Arbeiten aus dem Zool. 

 Instit., Wieu, t. iii, 1880. 



* " Ueber die Entwickelungsgeschichte der Malermuschel," ' Jenaische 

 Zeitschrift,' xi, 1878. 



