gS BIOLOGICAL LECTURES. 



relation to the future body axes, and by the fact that the large 

 cell D always lies at the posterior end of the animal. Compari- 

 son with other annelids shows that wherever this difference in 

 size appears, the large cell always lies at the posterior end of 

 the body, and from it are given off the cells 2d and ^d, which 

 are larger than the corresponding members of the same quar- 

 tette. In Unio, among the molluscs, a similar relation appears. 

 In other molluscs apparent exceptions occur, of which I will 

 speak later. 



Let us now turn to a consideration of equal cleavage among 

 annelids. For an explanation of this type I will take Podai'key 

 as the one whose development has been most completely stud- 

 ied. The four-cell stage of this form is shown in Fig. 2. All 

 the cells are equal in size, and the only orientation point which 

 presents itself is the " polar furrow." Observation of the living 

 Q.^g shows that this " polar furrow " remains constant in direc- 

 tion up to a comparatively late stage in the cleavage, when it 

 becomes possible to orient the ^g^ by means of other landmarks. 



In the next figure (Fig. 4) is shown the eight-cell stage, seen 

 from the side. Instead of a set of smaller micromeres, lying 

 on top of four larger macromeres, we have eight cells, all 

 exactly alike in size ; and only by means of the polar globules 

 are we able to distinguish the animal from the vegetative pole 

 of the Q^^. The "polar furrow " enables us to distinguish the 

 second from the first plane of cleavage ; but there is no differ- 

 ence in the size of the cells, as in AmpJiitrite, by means of which 

 one end of this furrow can be distinguished from the other. 



At the next division a second group of micromeres is formed, 

 and, by a division of the first group, four smaller cells, which 

 are the parent cells of the primary prototroch. These, exactly 

 as in Amphitrite, divide twice, and twice only, and form the 

 beginning of the larval locomotor organ, which becomes func- 

 tional at the completion of the sixty-four-cell stage. 



The cells of the second quartette, given off at the sixteen-cell 

 stage, are all of the same size, in contrast to what occurs in 

 Ajup/iitrite, where 2d is so much larger than the other members 

 of the same quartette. 



As in Amphitrite, a third quartette of ectomeres is given off, 



