No. 3.] THE CELL-LINEAGE OF NEREIS. 385 



it increases in bulk and ultimately passes, in part into the first 

 somatoblast, in part into the second. It would be interesting 

 to investigate carefully the nature and history of this peculiar 

 structure, but for the present I must pass it by. 



2. TJie Second Cleavage (Figs. 4% 4, 5). 



The second cleavage-plane coincides with the median plane 

 of the adult body. It divides the smaller blastomere {AB) into 

 approximately equal right and left halves {B and A respec- 

 tively), and the larger {CD) into unequal parts. Of these, the 

 larger {D), on the left side, is much larger than any of the other 

 three ; the smaller {C), on the right, is intermediate in size 

 between D and A or B. The precise mode of formation and 

 ultimate relation of these four cells must be accurately under- 

 sto.od, since they determine the orientation of the embryo 

 throughout the early development, and suggest some interest- 

 ing comparisons with various other animals. 



As in the case of many other eggs, the division of the smaller 

 cell takes place slightly in advance of the other, and the respec- 

 tive cleavage-lines of the two cells are not precisely in the same 

 position. The second cleavage may, in fact, best be treated, 

 not as a single cleavage of the Q.^g as a whole, but as consisting 

 of two rapidly succeeding divisions of the first two blastomeres. 

 Both divisions are of the same type as the first cleavage, the 

 furrow first appearing on the upper side and cutting thence 

 downwards. The smaller cell begins to divide about eighty-five 

 minutes after the fertilization. Starting from the upper side, 

 the furrow cuts downwards somewhat obliquely towards the 

 left side, and finally meets the first cleavage-line to the left of 

 the lower pole. The smaller blastomere is thus divided into 

 two parts {A and B), of which the left {A) appears to be slightly 

 the smaller when seen from above. 



During the division of AB, the larger blastomere {CD) begins 

 to divide in the same manner. At first, the furrow on the upper 

 side is nearly or quite a continuation of the cleavage-line between 

 A and B. As it travels downwards, however {cf. Figs. 4, 4^ 5), it 

 passes slightly to the right so as to meet the first furrow at a con- 

 siderable distance to the right of the cleavage-line between A and 

 B. Thus the first cleavage-line on the lower pole is divided into 



