No. 3.] THE CELL-LINEAGE OF NEREIS. 383 



A general view of the cleavage may be obtained from the 

 accompanying diagram or cytogenetic tree (p. 382), which repre- 

 sents accurately the genetic relations of the blastomeres up to 

 the complete establishment of bilaterality. The number of blas- 

 tomeres in the successive stages is given in the upper line ; the 

 succession of the divisions is shown by the vertical columns. 

 The record is without a gap up to the fifty-eight-celled stage. 

 Beyond this point the development of the embryo as a whole 

 cannot be fully represented in the diagram, on account of 

 increasing variations in the order of division of the individual 

 cells. Special diagrams showing the history of some of the 

 individual protoblasts will be given further on. 



III. Spiral Period of Cleavage 



(l to 38 blastomeres). 



I. The First Cleavage (Figs. 2* to 3). 



The first cleavage takes place about seventy-five minutes after 

 fertilization. The plane of division passes slightly to one side 

 of the polar cells, and divides the Qgg into two unequal parts 

 {AB and CD). The subsequent history shows that the first 

 cleavage plane is vertical and at right angles to the median 

 plane of the larval body, and at right angles to the prototroch. 

 The smaller of the two cells {AE) is anterior ("ventral," v. Wis- 

 tinghausen), the larger {CD) is posterior (" dorsal," v. Wisting- 

 hausen). 



The division is initiated by the appearance of a shallow, trans- 

 verse furrow on the upper side of the ovum, which gradually 

 deepens and ultimately cuts through the entire vitellus from 

 above downwards. The furrow is so placed as to divide the clear 

 polar area into unequal parts, about one-third of it passing into 

 the smaller cell and two-thirds into the larger. As the furrow 

 travels downwards, a number of interesting phenomena may be 

 observed. The furrow is at first very broad, with widely sepa- 

 rated margins, which only approach and come into contact when 

 the furrow has cut about one-third through the vitellus. As 

 they approach, the margins become irregular and put forth amoe- 

 boid processes, consisting mainly of clear protoplasm, though 



