52 The Ascidian Half-Embryo. 



plied by Castle * to the Ciona egg has been used for obvious 

 reasons. According to this s3^stem,now well known, each cell is 

 designated by a letter referring to the particular quadrant of the 

 four-cell stage from which it arose ; in addition it receives an ex- 

 ponent denoting the generation to which it belongs, and a second 

 exponent denoting its place in that generation, counting from 

 below upward. 



Detailed Description of Cleavage. 



A. Normal Cleavage. — The cleavage of the Molgula egg is pre- 

 cisely the same as that of Ciona and other ascidians, as far as it 

 has been followed. Therefore, it is unnecessary to discuss the 

 normal phenomena further than to emphasize a few of the facts 

 which are important in connection with the cleavage of the frag- 

 ments. 



The first and second cleavage-planes are meridional, while the 

 third is equatorial. An eight-cell stage results (fig. 1) which, 

 seen from the side, consists of two tiers of four cells each. The 

 upper tier is shifted anteriorly upon the lower, so that the poste. 

 rior upper cells are in contact with the anterior ventral cells- 

 This relation is constant, and characteristic of probably all as- 

 cidian eggs (Castle, loc. cit., p. 228). Passing to the 16-cell 

 stage, all the eight blastomeres divide. The spindle axes are in- 

 clined in such a manner that the anterior products of the anterior 

 cells (fig. 2 : B 52 , b 5 - 4 ) lie slightly below the median products ; 

 while the posterior products of the posterior cells lie slightly 

 above the other cells (fig. 2 : C 51 , c 5-3 ). When activity is again 

 resumed, the dorsal cells remain quiescent, while the ventral cells 

 segment, and a 24-cell stage results (fig. 3). After a period of 

 rest the dorsal cells pass into the same generation (sixth) with 

 the ventral cells, and a morula of 32-cells results. Then the ven- 

 tral cells divide at about the same time, while the dorsal cells re- 

 main quiescent, giving a 48-cell stage. 



Further details are unnecessary for our purpose. We empha- 

 size the fact that, beginning with the 16-cell stage, there is a well- 

 marked alternation of activity between the cells of the upper and 

 those of the lower hemisphere of the embryo. 



* Castle, W. E. The Early Embryology of Ciona intestinalis. Bulletin of 

 Mus. of Comp. Zool. Harvard. Jan. 1896. 



