No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 49 



same differences exist between the anterior and posterior lips, 

 they are now near together. But the mesoderm teloblasts no 

 longer lie immediately behind the posterior lip of the blasto- 

 pore ; they are separated from it by a wide space. It therefore 

 follows that the blastopore has closed from behind forwards. 

 How has this closure taken place .-' By tJie fo7"ward growth of 

 the ventral plate, which eventually meets the large cohunnar cells 

 marking the position of the anterior blastoporic rim (PI. V, Figs. 

 69 and 73). Thus a plate of cells is established, extending 

 from the hinder limit of the shell-gland to the anterior limit of 

 the blastopore. 



It is this plate of cells which becomes covered with cilia in 

 embryos of Anodonta, the action of which causes a rotation of 

 the embryo on its antero-posterior axis. In Unio such a rota- 

 tion does not take place. It has, hence, been concluded by 

 some (Rabl, No. 25) that the cilia are absent from this region. 

 Others assert their presence (Schierholz, No. 30). With com- 

 paratively high powers of magnification (X600) I have been 

 able to see very distinctly that this region is covered with 

 extremely fine and active cilia in embryos of Unio. Small par- 

 ticles within the Qg% membrane which came in contact with the 

 ventral plate were immediately swept away, and always toward 

 the mouth region. 



Considerable discussion has taken place as to the morphology 

 of this ciliated tract. Thus it has been supposed to repre- 

 sent a rudimentary velum. The last idea published is, that 

 they represent either the ventral or post-anal cilia of other 

 lamellibranch embryos (Korschelt and Heider, No. 51). The 

 question is of the morphology of the region bearing them. 

 The region is that of the ventral plate, which, as I have 

 already said, forms the foot and pedal structures, and, in addi- 

 tion, the post-anal region. The cilia in question, then, are 

 homologous with those on the ventral surface of other Mol- 

 luscan embryos. I shall have to postpone proof of this to a 

 still later section. 



A glance at Fig. 69 reveals the fact that just beneath the 

 anterior end of the shell-gland there are large cells provided 

 with large nuclei, each of which possesses a well-marked nucle- 



