572 ClfESSWEIJ, SUKAKKI^ 



that the head-kidney strand consists of a tliin thread of 

 cytophisni, at either end of which are the nuclei. A fine 

 lumen bejj'ins to appear in the middle about the second day; 

 this increases in size and works its way towards either end, 

 and by the middle or end of the third day the organ becomes 

 functional as the head-kidney, having- acquired an opening 

 into the proctoda3um. 



G. The Co-ilomesoblast. 



Towards the end of gastrulation, and after the period when 

 the ectomesoblast has already appeared in the blastoccel, two 

 large cells are seen side by side in the ventral lip of the 

 blastopore. In surface views they seem to lie more in the 

 ventral ectodermic plate than in the endoderin. From 

 sections, however (Text-fig. 17, me.), they are seen to be part 

 of the endoderm at its point of junction with the ventral 

 plate. They are not free in the segmentation-cavity, and 

 during the coui'se of invagination they come to lie in the wall 

 of the primitive ai'chenteron. They finally rest in the anal 

 end of this structure, where, at much later stages, by a series 

 of rapid divisions, they give rise to a number of cells which 

 push out into the blastocoel and form the mesoderm bands. 

 They are, therefore, the coelomesoblast cells. 



In the stage represented in section in Text-fig. 17 they ;ire 

 nsually seen in the ventral lip of the blastopore nndergoing 

 division. The fate of the smaller of the resulting daughter- 

 cells I have been nnable to determine, but I believe they 

 represent the small cells forming part of the wall of the 

 archenteron in Podarke. The larger of the two cells 

 becomes the coelomesoblast. As development advances they 

 are carried back in the wall of the archenteron, and do not 

 lie free in the blastocoel till a later stage. In late stages 

 they are seen in the anal end of the archenteron as in Text- 

 figs. 19-25 [cd'. m.) ; here they always project slightly from 

 the gut-wall. 



After their division, as shown in Text-fig. 17, the various 



