EMBRYONIO FISSION IN OYOLOSTOMATOITS POLTZOA. 21 



The primary aperture is still present ; the distal thickening of 

 the tentacle-sheath being already distinct^ and showing a dif- 

 ferentiation of an external epithelial layer, and a more inter- 

 nally placed mass of nucleated protoplasm. 



It is very difi&cult to make out with certainty the later 

 history of the aperture. In stages previous to that at which 

 the solid follicle has its maximum size, the connection of the 

 primary aperture with the distal wall of the ovicell seems to 

 be completely lost in many cases. It is perhaps the case that 

 the original invagination remains connected wiih the distal 

 wall of the ovicell by a thin cord of cells which is not easily 

 seen in sections — accounting for the apparent discontinuity 

 between endocyst and apertural invagination which is fre- 

 quently remarked ; and that, later, this cord shortens, bring- 

 ing the invagination once more nearer to the distal wall of the 

 ovicell, where its aperture once more opens out widely. 



In fig. 13, representing the aperture of an ovicell in which 

 traces of vacuolation are beginning to appear in the follicle, 

 the primary aperture still opens to the exterior ; although, in 

 accordance with what has just been said, its opening would 

 probably not have been discernible at a somewhat earlier 

 stage. The formation of the tubular definitive aperture has 

 progressed, and the valve is complete. The differentiation of 

 the thickened part of the tentacle-sheath into two kinds of 

 cells, alluded to in the description of the last figure, has 

 advanced a stage. 



In fig. 9 the tubular aperture is practically complete. At its 

 end is seen an invagination which I regard as the remains of 

 the primary aperture, but which has now become disconnected 

 from the thickened part of the tentacle- sheath. 



The history of the aperture is thus, according to what I 

 believe I have made out, as follows : — During the calcification 

 of the distal end of the ovicell, the primary aperture, which at 

 first opened in the middle of the mouth of the funnel, becomes 

 shifted nearer the " back " wall of the ovicell, into the position 

 where the tube of the ovicell is to be formed. The aperture is 

 thus not closed by the calcification of the ovicell, but finally 



