Mosaic Development in Ascidtan Eggs. 213 



Cerebratulus before maturation may give rise to entire larvae; 

 whereas this is not usually the case after maturation and fertiliza- 

 tion, the potencies of the substances at the animal and vegetal 

 poles being different. It is evident that during the concentric 

 stage of localization section ot an egg in any plane would leave 

 samples of all the ooplasmic substances in each piece; in the stage 

 of polar-radial localization any section of the egg parallel with the 

 egg axis would leave samples of all the ooplasmic substances in 

 each piece; in the bilateral stage, only the right or left halves would 

 contain parts of all the substances. Probably one important rea- 

 son why parts of eggs may give rise to whole embryos in some 

 cases and not in others may be found in the fact that at the time 

 of the experiment the form of localization may have been concen- 

 tric in some cases, radial in others and bilateral in still others. 

 (See Boveri, '01; Wilson, '04^) 



3. Cleavage and Localization. 



In previous publications ('05s '05-) I have pointed out the fact 

 that localization precedes cleavage in the ascidian egg and that the 

 localization pattern does not closely coincide with the cleavage 

 pattern. On the other hand there is normally a constant relation 

 between particular cleavage planes and the various ooplasmic 

 substances. The first cleavage always lies in the median plane 

 and bisects all the ooplasmic substances; the second is transverse 

 to the median plane and separates the yellow crescent from the 

 gray one; the third cleavage is at right angles to the two preceding 

 ones and separates the clear ectoplasm of the ventral hemisphere 

 from the different substances of the dorsal hemisphere. Probably 

 in no other animal is the cleavage so constant and so perfectly 

 bilateral as in the ascidians and yet even here the position and 

 direction of the cleavage planes is less constant than the form of 

 localization. 



Among annelids and mollusks, as is well known, the cleavage is 

 typically spiral and in many cases it is radially symmetrical. 

 This radial symmetry of cleavage does not indicate, however, that 

 the localization of ooplasmic substances is also radially symmet- 

 rical, for in some cases this localization is known to be bilateral 

 and this is probably true in all cases. (See Conklin,'o5S pp. 90-92.) 



The relation of the cleavage planes to this bilateral organization 



