ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 39 
48-cell stage to designate the actual right and vzce 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. 
В. CLEAVAGE or THE EGG; First то SEVENTH GENERATION OF CELLS (1-64 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 Crona 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. Certainly 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 165. Тһе different generations of cleavage cells are 
separated from one апе: by intervals about as follows : 
Fertilization to first cleavage ( 1-2 Cells), . 40 min, 
First to second cleavage [X nA. oU € 
Second to third cleavage LX. ^. 30. * 
Third to fourth cleavage [go c s. ; А 
Fourth to fifth cleavage ( 16-32 “ ), 2n 
Fifth to sixth cleavage (32-604 * ), . ZR ын 
Sixth to seventh cleavage ( 64-112 * ), . > 20 * 
Seventh to eighth cleavage (112-218 “ ), : о * 
Eighth cleavage to young tadpole stage (fig. 163), : 2 hrs. 
1. Furst 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 egg and the posterior pole (figs. 91, 92, 93), the cleavage spindle 
vibe rapidly and moves inward from the posterior pole until it lies almost 
hese figures are based upon the study icf eggs laid and fertilized at 5 p.m. and put up at inter- 
vals dede until 11 p.m. Іп each of these lots eggs were found in several different stages and the 
results can be pe as only approximately correct. 
