425 



differentiation takes place in the tissue in situ or is formed by a solid 

 ingrowth is question which would be very difficult to answer in an em- 

 bryo in which cell boundaries are as little marked they are here. The 

 appearances however all favour a differentiation in situ. 



In the embryo figured in Fig. 4 the proctodaeum is already formed 

 and has a lumen opening on the posterior wall of the elongating embryo. 

 The ventral slit can be traced round the posterior end of the embryo 

 up- to the proctodaeal opening. 



Both mouth and anus therefore open into the extreme ends of this slit. 



Interpretation of this mode of formation of the 

 mesendoderm. 



The mode of formation of the mesendoderm in Holopedium by 

 immigration of cells from the mid-ventral line of the blastoderm is simi- 

 lar to that described by S a ma s sa for several Cladoceran species, but 

 with some noteworthy differences in detail. Samassa finds that in the 

 species examined by him the mesendoderm arises by a process of immi- 

 gration of cells along the mid-ventral line. His figures of this stage 

 show a striking similarity to my figure 3 , with the important difference 

 that in Holopedium the ventral part of the blastoderm has already be- 

 come many layered, while in Samassa' s species it is still one layered. 

 According to S am as sa' s figures, after the mesendoderm has migrated 

 into the interior of the embryo, the ectoderm is left intact. As already 

 described in Holopedium the immigration of the strand of mesendoderm 

 tissue into the interior of the embryo leaves its place occupied by a 

 conspicious slit filled by a tissue much less dense than the ectoderm on 

 each side of it. 



This slit is to be regarded as a blastopore of the primitive elongated 

 type. It stretches underneath the whole length of the mesenteron and 

 embraces the openings both of the mouth and anus. From the edges of 

 the slit arise the nerve cords. These of course meet in front of the slit 

 in the cerebral ganglion. I have not found a post-anal commissure at 

 any stage (Samassa however found it in the adult of Sida crystallina). 



Thus except for the absence of a post-anal commissure we should 

 have in Holopedium a condition similar to that found in Peripatus. The 

 method of closure of the blastopore is such a modification of the primi- 

 tive method as we might have expected where the presence of a large 

 mass of yolk filling the entire blastocoele causes invagination to become 

 immigration. The closure does not take place by folding over the edges, 

 but the immigration of tissue to form the mesendoderm is not quite 

 complete, a plug of loose tissue being left behind in the blastopore after 

 immigration is ended. This tissue, which remains for a long time in a 



