ON THE flASTTlULATIOX IN PETTIOMYZON. 25 



tlio projoetod part. Now it follows that the cells composini;; the 

 roof of the arehenterie pocket first formed are those which 

 formerly gave rise to the conical eminence, while its floor is re- 

 presented by the macromeric mass which had lain ventral to the 

 eminence. 



The layer constituting the arehenterie roof shows an 

 e]tithelial character {Figs. 17 and 18, mac. ep.) ; it is, however, 

 not difhcnlt to distinguish from the regular cylindrical epithelium 

 of the outer layer. It looks irregular because it is composed of 

 tall cells which are variable in shape and length and are thickly 

 loaded with yolk-granules ; further it is distinguished by the 

 nuclei of the cells beino- laro'er than those of the outer 

 layer cells. The outer regular and the inner irregular epithel- 

 ium pass over into each other at the dorsal lip of the 

 blastopore. On the other hand, the inner layer is to be traced 

 uninterruptedly into the outermost row of the macromeres, which 

 is partly invaginated but is in the greater part still ex^oosed. The 

 irregular epithelium arises without doubt by delamination, so to 

 speak, from the subjacent cells. The row of cells forming the 

 floor of the segmentation cavity has nothing to do Avith the 

 epithelial layer in question ; it always shows its original sharp 

 contour. On the contrary, in front of the anterior end of the 

 invaginating pocket, there is always seen some disturbance of 

 cells, and some detached cells are seen projecting into the 

 segmentation cavity {Figs. 17-19, •::••::• and Figs. 25 and 26, •::••:> ). 

 This appearance is due, it seems to me, merely to the fact that 

 here the outermost row of macromeres is being split off" from the 

 underlying crowd of cells, to be turned into the arehenterie 

 roof. 



Passing now to a little further advanced ovum {Fig. 19), the 



