98 HOLMES. [VOL. I. 



the upper or fourth quartette. At about this time the large 

 mesoblast cell M divides ; the two resulting mesomeres lie 

 partly pushed into the cleavage cavity so that only a small por- 

 tion of each appears at the surface. They become entirely cov- 

 ered over by the ectoderm at about the sixty-four-cell stage. 

 With the division of ectomeres and the mesomere the number 

 of cells has reached forty-nine. This stage marks another 

 resting period. The egg has now seven entomeres, two meso- 

 meres, and forty ectomeres, the first and second generations 

 of ectomeres each containing sixteen cells, while the third 

 quartette is composed of eight. Up to this time all of the 

 ectodermic cells of a quartette have undergone division at 

 approximately the same time. The cleavage of certain cells 

 of the third quartette which now takes place introduces an ex- 

 ception, in this respect, to the previous regularity of cleavage. 

 The anterior cells of the lower tier of the third quartette, ^b~ 

 and 3<r 2 , divide bilaterally, and, at nearly the same time, the two 

 posterior cells of the upper tier of the same quartette, 3^' and 

 3<-/', undergo a division which is likewise bilateral. The large 

 cells 3 2 and ^d~ remain for a long time undivided. The cells 

 2a 1 ' 2 , 2b 1 ' 2 , etc., and 2a 2 '\ 2b 2 ' 1 , etc., next undergo a laeo- 

 tropic division, and, at the same time, the three cells of the 

 fourth quartette, 4^, 4$, and 4^, divide in an equatorial direc- 

 tion. Soon afterwards the cleavage of the upper anterior cells 

 of the third quartette, 3^' and 3^', occurs, followed by the divi- 

 sion of the cells forming the bases of the arms of the cross 

 (la 1 ' 2 , \b 1 ' 2 , etc.). The egg now consists of seventy cells, - 

 twenty cells of the first quartette, twenty-four of the second, 

 fourteen of the third, eight of the fourth, and four small cells 

 at the vegetative pole. From this stage on the cells of the 

 third quartette which have not hitherto kept pace with those 

 of the first and second undergo rapid divisions. 3# I-I , 3 I-2 , 

 3c/ I-I , and $d 1 ' 2 each give off a small cell toward the ventral 

 pole, and ^b"\ 3& 2 ' 2 , $c 2 ' 1 and ^c 2 ' 2 likewise give off a small 

 cell in the same direction at the anterior side of the egg ; thus 

 arise four pairs of small cells, two anterior and two posterior. 

 Soon all of the upper cells of the third quartette divide in the 

 same direction as before. There thus results in the anterior 



