64 CONK LIN. [Vol. XIII. 



history of the whole &gg up to the point, Fig. 22, where the 

 segregation of the layers is practically complete, and then to 

 deal separately with the history of each of these layers ; accord- 

 ingly, I shall describe here the first division of the second 

 quartette, which occurs before the separation of the mesoblast. 



Very soon after the formation of the third quartette the 

 second quartette divides. Figs. 18 and 19. It is not possible 

 during the nuclear division to tell which end of the spindle 

 lies at the higher level, though the right end lies nearer the 

 mid line of each macromere, Fig. 19, and after the cell division 

 it is seen that the right moiety overlaps the left, Fig. 20. 

 The spindles are, therefore, arranged in a right-wound spiral, 

 and the division is dexiotropic. The two moieties are about 

 equal in size, though the right one seems the larger because 

 it overlaps to a certain extent the left. 



At this stage there are twenty micromeres and four macro- 

 meres. The micromeres are arranged in a plate, the rounded 

 corners of which lie in the furrows between the macromeres. 

 Fig. 19, Diagrams 4 and 5. The centre of the plate is formed 

 of four apical cells and four turret cells, which are the deriva- 

 tives of the first quartette. These eight cells form a rect- 

 angular plate with its corners in the furrows between the 

 macromeres. Around this central plate of eight cells is a belt 

 of twelve cells, consisting of eight cells derived from the second 

 quartette and four cells of the third quartette ; these cells we 

 shall call the belt cells. In Fig. 20 it is seen that the apical 

 and turret cells overlap the belt cells, so that the micromeres 

 are arranged like the shingles on a roof. The apical cells do 

 not overlap the turret cells ; in the division of the second quar- 

 tette, as has been explained, the right moiety overlaps the left; 

 while underlying all of these is the third quartette. 



The first division of the second quartette occurs in essen- 

 tially the same way, though subject to certain variations in time, 

 in all cases in which the cleavage has been carefully studied, 

 with the single exception of Neritina. Blochmann ('81) asserts 

 that in this animal the cleavage is not dexiotropic, as is true 

 elsewhere, but is laeotropic. This difference in itself might 

 seem to be of little importance, but since it profoundly modifies 



