The Development of Ischnochiton. 641 



posterior gap, is secondary. And it is reasonable to suppose 

 that the post-trochal cells so intimately related to the trochoblasts, 

 and forming a band immediately below the functional prototroch, were 

 originally present in all quadrants. Indeed at the present time the 

 right post-trochal in the posterior quadrant forms as elsewhere but I 

 shall attempt to show that there is an anachronism, in the posterior 

 quadrant, of the cleavage which produces the left post-trochal cell. 



In quadrants Ä, B and C, the first cleavage of the second quartette 

 produces an upper and lower product, the former producing at the 

 next cleavage the secondary trochoblast and right post-trochal cell, 

 the latter at its first division giving rise to the left post-trochal cell 

 and a blastomere which at its next cleavage produces a stomatoblast. 



In the posterior quadrant the first cleavage oi 2d is identical to 

 that occurring in the other quadrants and the cleavage of the upper 

 cell produces post-trochal and tip cell (secondary trochoblast), but 

 the lower cell {2 d 2) at its first cleavage produces a secondary stomato- 

 blast, the cell corresponding to the left post-trochal arising at the 

 next division. (Compare Figs. 18 and 19 and 26 and 27.) Thus in the 

 posterior quadrant there is an acceleration in the formation of the 

 secondary stomatoblast and an omission of the cleavage which forms 

 the left post-trochal cell which may be accounted for I believe in the 

 following way. Originally all the members of the fourth quartette were 

 entoderm, were of the same size, formed at the same time, and were 

 part of a perfectly radial organism. Modifications arose m 4d (meso- 

 blast), which now assumed a role different from the remaining fourth 

 quartette cells and the cleavage which produced it occurred earlier 

 than in the remaining fourth quartette. Now the secondary stomato- 

 blasts in A, JB and C arise at the time the fourth quartette cells 

 form, and the same thing occurs in quadrant D. But the accelerated 

 cleavage oi 4d occurs about the time when the left post-trochal cells 

 form in the other quadrants, and hence, if the posterior stomatoblast 

 arises at the time when 4d forms the cleavage which originates the 

 left post-trochal cell in this quadrant must be omitted. 



Thus it seems to be of more importance that the stomatoblast 

 be formed at this time than that the post-trochal cell be produced. 

 In other words the correlation between the stomatoblast and the 

 mesoderm appears to be closer and perhaps of more vital importance 

 than that existing between the post-trochal cell and the prototroch. 

 It clearly seems to an onlooker that if the post-trochal cell arose at 

 the same time in all quadrants, the stomatoblast arising at the next 



