No. 3-] EMBRYOLOGY OF UNIO COMPLANATA. 575 



note that, according to Lang,i all of the second generation of 

 micromeres in the Polyclad Discocelis contribute to the meso- 

 blast, and that the larval mesoblast of Unio is part of one of 

 the second generation. 



Gastrulation. 



I cannot state exactly how many cells enter into the forma- 

 tion of the entoderm, but whatever their number they are the 

 products of A, B, C, and D. The third generation of micro- 

 meres is purely ectodermic. Gastrulation takes place at a stage 

 when about twenty shell-gland cells are present. The invagi- 

 nation is never very deep, and in a superficial examination of 

 whole embryos might easily escape observation altogether. 

 That it has escaped observation hitherto is attributable to this 

 fact. In section it is, however, very obvious (Fig. 5). Posterior 

 to the blastopore in the segmentation cavity lie four cells, 

 products of M, and anterior to it four cells, products of Y 

 (Fig. 5). The blastopore occupies the posterior region of the 

 body, and is afterwards occluded by products of X (Fig. 6). 

 The anus arises later in the same area. 



The oesophagus arises in the area described as oral plate 

 (" Mundschild ") by Flemming (Fig. 6, oes.). In some glochidia 

 of Anodonta it is already in communication with the digestive 

 tract, but as a rule the communication is not effected till 

 post-embryonic stages, during the parasitic attachment of the 

 glochidium to a fish. 



Shell and Foot. 



Interpreting the entoderm invagination of Rabl as shell- 

 gland, I can only corroborate his account of the formation 

 of the shell. 



The cells which lie between the blastopore and the shell- 

 gland are derivatives of X (Figs. 5 and 6). These cells 

 grow past the blastopore in later stages and anterior to it as 

 far as the mouth. When the whole ventral region of the 



1 Lang, Die Polycladen, p. 332. Monographie 11. Herausgegeben von der 

 Zool. Stat, zu Neapel. 



