l62 Edwin G. Conklin. 



B^-^ into B^^ and B*'^ is almost at right angles to its normal 

 direction. In other cases, as is shown in Fig. 24, this cleavage is 

 normal in direction, and I am, therefore, of the opinion that the 

 condition shown in Fig. 17 and the later stage of this same egg 

 shown in Fig. 19 may be due to some slight injury to the developing 

 half of this egg. In Fig. 18, which is a dorsal view of the same egg 

 in the same stage as Fig. 17, the cells A**-^ and A"^ have moved in 

 toward the median plane as compared with Fig. 16, though in this 

 respect, also, the corresponding stage shown in Fig. 24 is quite 

 normal. This shifting of the anterior dorsal cells toward the 

 median plane is shown again at the next cleavage (the sixth), of 

 this egg. (Fig. 20.) 



The seventh cleavage, which is shown in Figs. 25 and 26, is also 

 normal except for the direction of a few of the divisions. The 

 cells which constitute the yellow and gray crescents are in all 

 respects like the right half of a normal egg. However the position 

 of the cells A^-^ and A^-, Fig. 25, and the direction of division in 

 several of the ectoderm cells shown in Fig. 26 are not quite normal. 



In conclusion therefore it may be said that the cleavage of one 

 of the blastomeres of the 2-cell stage or of the right or left blasto- 

 meres of the 4-cell stage, is like that of the corresponding half of a 

 normal egg, except in minor details. Even these minor differences 

 are not always present and when they are they do not alter the 

 localization of the ooplasmic substances. In every case the dis- 

 tribution of the yellow, the gray and the clear substances to the 

 different blastomeres is the same as in the right or left half of a 

 normal egg; the cells of the yellow crescent, for example, form only 

 the right or left half of a normal crescent, and the same is true of 

 the gray crescent and of the other substances of the egg. Even 

 the small amount of yellow protoplasm which is found around the 

 nuclei of the posterior ectoderm cells b^-. Fig. 13, is perfectly 

 normal in its occurrence and subsequent distribution. 



I have elsewhere ('05^) shown that the localization of different 

 ooplasmic substances in the ascidian egg precedes cleavage and 

 that cleavage and localization are here relatively independent of 

 each other; these experiments show that in both cleavage and 

 localization the development of the right or left half of an ascidian 

 egg is a "mosaic work," for the slight amount of regulation, which 

 is manifested in the changes in the direction of certain cleavages, 

 and the consequent closing of the embryo in no wa}' alters the 



