DEVELOPMENT OF THE EUROPEAN OYSTER. 3 4^ci 



assumed, in consequence of an invagination at the lower 

 side, a reniform shape (fig. 3, the lower part is turned up- 

 wards) 



By examining somewhat older embryos by section (fig 4), 

 it becomes clear that the layer of hypoblastic cells is a little 

 invao-inated, and so a gastrula has arisen. Since there is 

 no segmentation cavity one cannot speak of a true invagina- 

 tion, and it is difficult to say if we have to do with an em- 

 bolic or an epibolic gastrula. The last form seems to be 

 characteristic for the other marine Laniellibranchiata. In- 

 deed RabP and others have already shown that these two 

 apparently fundamental different ways of gastrula formation 

 are connected by a series of transitions. 



The oyster embryo shows in this stage the remarkable 

 peculiarity that not only the vegetative pole has an invagina- 

 tion, but that besides there is to be found a very evident pit 

 at the opposite pole, a little way from the top. Examining 

 the embryo from the side this invagination is quite striking 

 (fig. 5, sk), and an optical section (fig. 4) teaches us that it 

 has arisen by epiblastic cells, which have somewhat inva- 

 ginated themselves inwards. By further development there 

 arises here a pouch formed by high cylindrical cells, 

 with a narrow lumen, whose blind end is turned to the 

 dorsal pole of the embryo, whereas its opening is directed 

 transversely to the longitudinal axis of the embryo (tigs. 7 

 and 8). Surely this pouch is nothing but the shell-gland, as 

 is shown by the examination of older stages. The assertion 

 of Fol^ that in Ostrea the shell-gland is not a true in- 

 vagination, but rather a feeble excavation of the thickened 

 epiblast, is therefore not quite exact, and probably founded 

 upon the examination of older larva? (observed by Salensky), 

 in which the invagination afterwards everts and flattens 

 out gradually, like that of other Mollusca embryos. The 

 shell-gland, as we know, was first discovered^ in the Mollusca 

 cephalophora, and was afterwards observed also by Ray 



* " Entwicklimg der Tellerschuecke," ' Morphol. Jabrbuch.,' Bd. v, p. 

 601. 



2 " Etudes sur le Developpemeut des Mollusques," ' Archiv. de Zool. 

 Experim,' t. iv, p. 180. 



^ [Dr. Horst is mistaken in stating that the shell-gland or prsecouchjlian 

 invagination was first observed in Mollusca ceithalopliora. It was discovered 

 by Laukester in Pisidium in 1871, being previously altogether unknown, 

 and its occurrence in the embryo of that Lamellibranch and of Aplysia, 

 announced by him in the ' Annals of Nat. Hist.,' February, 1873, seven 

 years before Dr. Hatschek's publication, and two years before that of Fol. 

 In 1874 Lankester published in this Journal and in 1875 in the 'Phil. 

 Trans.,' figures of the shell-gland in Lymnseus, Neritina, and Aplysia, as- 

 well as in the Lamellibranch Pisidium.— Ed. « Q,. J. Mic. Sci.'] 



