No. 4.] CELL LINEAGE OF PODARKE OBSCURA. 



I 99 



beginning of the process.) Subsequently, however, these 

 nuclei come to the surface and divide, forming the fifth group 

 of micromeres. The eight cells thus formed invaginate, form- 

 ing with the three members of the fourth quartette an invag- 

 inating plate of eleven cells, and the blastopore closes rapidly, 

 closure being effected largely, if not entirely, by the division of 

 the descendants of the third group of micromeres. 



For purposes of comparison I have made a few observations 

 on other forms with equal cleavage. 



Lepidonotus sp. 1 



In Lepidonotus the bilateral cross and the small cell (.r 1 ' 2 ) 

 appear exactly as in Podarke. These structures were over- 

 looked by Mead, 2 who was also misled, by the appearance of 



a"* 



ap 



FIG. 4. Cross when first formed, seen from animal pole. Some of the intermediate cells 



are dividing. Sixty-eight cells. 



a " pseudo-invagination " similar to that of Podarke, into the 

 statement that no fifth group of micromeres are formed. Here, 

 as in Podarke, the nuclei migrate to the surface, and the micro- 



1 The most extended observations previously published on equal cleavage in 

 annelid eggs are those of Mead on Lepidonotus. To Dr. Mead's generosity I am 

 indebted for preparations of some of the later segmentation stages. 



2 L. c., p. 267. 



