Mosaic Developnie72t iii. Ascidian Eggs. 203 



or not must be found in the study of those organs which do not he 

 along the median plane. 



a. Neural Plate and Sense Organs. 



Chabry says that he never saw a partial embr}^o in which the 

 neural plate had invaginated; on the contrary the nervous system 

 always remains spread out in the form of a layer or plate; this 

 plate occupies the face of the embryo which is morphologically 

 median in position (its normal location), while the sense spots 

 consist of pigmented cells which are superficial in position and 

 which lie near the base of the tail. This agrees \&ry closely with 

 my observations, though I have frequently seen the neural plate 

 invaginate by an irregular process. The eye is said by Chabry 

 to be formed on the right side normally, but the fact that it may 

 appear in the left half embryo leads him to conclude that its rudi- 

 ment exists in the left half of the ezz also. He thinks that the 

 otolith comes only from the right posterior cell. I have not 

 determined the exact cell origin of the sense organs in the normal 

 larva, but in the partial larvae they are formed only from the 

 anterior quadrants and from either the right or left sides. I have 

 not been able to distinguish between the eye and the otolith in 

 the partial embryos of Cynthia. 



Driesch says nothing of the neural plate nor of the manner in 

 which the nervous system is formed in his small larvae, though he 

 mentions the fact that "the sense vesicle with the eye and otolith 

 are not formed in the typically clear manner characteristic of the 

 normal development." He found the eye spot almost always 

 present, the otolith very seldom and he concludes that it makes no 

 difference in the presence or absence of the sense organs whether 

 the embryo has developed from certain cells of the 4-cell stage 

 rather than from others. Since Driesch expressly states that he 

 never raised a quarter embryo beyond the stage of his Fig. 16, at 

 which stage the sense organs have not appeared, and since neither 

 his figures nor descriptions give any evidence that he has distin- 

 guished anterior or posterior quadrants from right or left ones, it 

 would be interesting to know how he could determine that sense 

 organs might be formed from any quadrant of the egg — a result 

 entirely contrary to my observations. 



