EMBRYOLOGY OF CLEPSINE. 265 



close adlierence of the two segments seems to be relaxed. 

 The plane of division passes to the right of the vanishing spot 

 from the centre outwards. By the time it has reached the centre 

 of this segment (fig. IS, 5 h. 10 min.) the two inner angles of 

 the cleavage walls have fallen together, the advancing end of 

 the cleavage groove assuming a rounded form. About this 

 time the cleavage of the smaller segment begins^ advancing, as 

 before, from the centre outwards. The second division severs 

 the smaller segment into nearly equal parts [a and c), but cuts 

 off only a third or a little more of the larger segment (6). At 

 its completion (lig. 19, 5 h. 30 min.) the appearance of the t^vo 

 segments a and c reminds one of the condition presented by the 

 first two segments in fig. 15. The inner angles are closed, while 

 the outer are still far apart. The same phenomenon recurs as 

 often as the cleavage appears on one side sooner than on the 

 other. 



(ft) Formation of the first four Ectoblasts. — Rathke entirely 

 overlooked the formation of these ectoblasts. Grube (, ^^ J 

 speaks of small " parietal spheres " (" Wandungsballen "), which 

 he supposed were formed in the interior of the blastomeres 

 (Huxley),' and afterwards ejected from the "active pole''^ (oral 

 pole). Grube evidently mistook nuclear bodies for these ecto- ■ 

 blasts, and probably confounded the latter with polar globules, 

 for he saw one of these " parietal cells " at the close of the first 

 division, lying between the two hemispheres on the oral pole. 

 This body could not have been an ectoblast, since these cells 

 arise after the second meridional cleavage. 



Robin (xTr^i-fT-o) asserts that these small cells arise as buds 

 from the three blastomeres a, c, and b, the fourth and larger 

 blastomere {x) producing none. The first two are produced 

 simultaneously by the two opposite blastomeres a and b, the 

 second two simultaneously by b and e. In the same way Robin 

 derives the four ectoblasts in Nephelis {-r^\i'*^-o) from three of 

 the primary blastomeres, without the participation of the fourth. 



Biitschli {-^"^^), on the other hand, supposed that each of the 

 four large spheres produced one of these ectoblasts ; but, as he 

 did not follow the process of this development on the living egg, 

 he was inclined (oViy) later to yield his opinion to that of Robin. 

 As Robin is certainly wrong in respect to Clepsine, I am inclined 

 to accept provisionally Biitschli's earlier opinion, inasmuch as it 

 is in perfect accord with what happens in Clepsine. 



About one hour after the completion of the two meridional 

 divisions, the two lateral blastomeres, a and b, are found to be 



' Tliis term is used with reference to the four primary cleavage spheres 

 alone. 



