Mosaic Development in Ascidian Eggs. 205 



formation of a tail is dependent upon the development of a noto- 

 chord. 



c. Muscles and Mesenchyme. 



Chabry paid no particular attention to the number and location 

 of the muscle cells in his partial larvae, though he frequently 

 speaks of their presence as being proved by the twitchings of the 

 tail; these movements are less energetic than in normal larvae and, 

 as a consequence, partial larvae do not escape from the egg mem- 

 branes. Driesch also found that partial larvae rarely hatch, 

 probably because of their weak muscular movements, but he, too, 

 paid no attention to the number and position of the muscle cells. 

 Owing to the brilliant color of these cells in Cynthia thev are 

 recognizable at all stages; in the partial larvae thev are found only 

 along one side of the notochord, where they form the characteristic 

 three rows of cells, whereas the muscle cells of the opposite side 

 are entirely lacking. In the oldest larvae a tew of the muscle cells 

 extend around the end of the notochord to the side on which they 

 were lacking. I have not been able to determine whether the num- 

 ber of muscle cells is actually increased during this process or 

 merely rearranged, but I believe that the whole process consists 

 in the moving of certain cells over to the side on which thev were 

 lacking, without any increase in their number. This is part of 

 that process of regulation which begins with the rounding up of 

 the survivms blastomere after the other one has been killed. In 

 fact, this very extension of the muscle cells around the end of the 

 notochord begins in this rounding up of the surviving blastomere 

 and in that slight change in the direction of division which causes 

 the median cells of the yellow crescent to lie nearer the middle of 

 the first cleavage plane than in the normal egg. (Fig- I5-) 



Chabry found (p. 308, Fig. 132) only one atrial invagination 

 and one organ of fixation (papilla) in right or left half embryos. 

 Driesch did not determine the number of atrial invaginations but 

 he does call attention to the fact that but one papilla is present in 

 embryos from isolated blastomeres. I have not observed the 

 formation of the atrial invaginations or of the papillae in Cynthia; 

 even in the normal larvae they are mconspicuous at the time of the 

 metamorphosis and I have not studied them before that period- 

 However, the areas of trunk mesenchyme in which the atrial 

 invaginations appear, are conspicuous areas of clear, slightly 



