ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 39 



48-cell stage to designate the actual right and vice versa. In my modification of his 

 system this substitution of one side for the other will be indicated only by the pres- 

 ence or absence of a line under the letter. In all stages later than the 48-cell stage 

 I continue to use lower case letters to designate cells of the animal or ectodermal 

 hemisphere, and capitals for those of the opposite hemisphere, whereas Castle 

 reverses this rule. 



B. Cleavage of the E<;g; First to Seventh Generation of Cells (1-G4 Cells). 

 Although the details of the early cleavages of the ascidian egg have been 

 treated at considerable length by previous writers on this subject, I have deter- 

 mined to present the subject here, cleavage by cleavage, both because my results 

 differ in many respects from the conclusions heretofore reached and because I wish 

 to call attention to certain features of these cleavages which have not as yet been' 

 noticed. I shall shorten the account wherever possible by references to previous 

 work. 



As is well known, ascidian eggs develop with great rapidity; there are cer- 

 tainly few other eggs which develop so rapidly. Both Ciona and Cynthia reach 

 the fully formed tadpole stage in about twelve hours, while Molgula reaches this 

 stage in not more than eight hours after fertilization. Certainlv there are few things 

 more wonderful than the origin of a complex animal. of a chordate , from an egg 

 in the short space of from eight to twelve hours ! The portions of this brief period 

 devoted to the different stages of development are interesting and suggestive. In 

 Cynthia about 40 minutes elapse between the fertilization and the appearance of 

 the first cleavage , furrow ; about 140 minutes between the first cleavage and the 

 beginning of gastrulation ; and about 140 minutes between this stage and the young 

 tadpole stage shown in figure 103. The different generations of cleavage cells are 

 separated from one another by intervals about as follows : 1 



Fertilization to first cleavage ( 1-2 Cells). 



First to second cleavage 



Second to third cleavage 



Third to fourth cleavage 



Fourth to fifth cleavage 



Fifth to sixth cleavage 



Sixth to seventh cleavage 



Seventh to eighth cleavage (112-218 



Eighth cleavage to young tadpole stage (fig. 163), 



1. First Cleavage; 1-2 cells. (Figs. 18-27, 96-100, 177. 178. 181.) 



After the meeting of the germ nuclei, which occurs about midway between the 



center of the ci'i and the posterior pole (figs. 91, 92, 93), the cleavage spindle 



develops rapidly and moves inward from the posterior pole until it lies almost 



1 These figures are based upon the study of eggs laid and fertilized at 5 p.m. and put up at inter- 

 vals thereafter "until 11 p.m. In each of these hits eggs were found in several different stages and the 

 results can be aecejitr.l u- only approximately correct. 



