594 WILSON. [Vol. VIII. 



cleavage the third division is vertical in all four, as shown 

 in Fig. no. A second somewhat similar case is shown in 

 Fig. 113. On the left side of the middle line the division 

 corresponds precisely with that of an isolated ^ blastomere. 

 The right side, however, corresponds with one-half of Fig. 114 

 (allowing for a slight displacement of the cells). 



These facts are of great importance, for they demonstrate 

 that even a slight displacement of the blastomeres in the 2- 

 celled stage causes a change in the form of cleavage, such that 

 the blastomeres of the half-embryo cannot be ideritified indi- 

 vidually with those of a normal embryo half. The normal 

 embryo develops as a nnit ; if it be disturbed in the 2-celled 

 stage, this tmity is destroyed and two new units are established. 

 In the transitional forms the new units show, as it wei^e, remi- 

 niscences of their parentage. 



It would be interesting to determine whether the isolated 

 half-embryos ever show such reminiscences. The case illus- 

 trated by Figs. 104 and 105, where the second division of the 

 isolated ^ blastomere is unequal, may perhaps be so regarded ; 

 but only in respect to the arrangement and relative size of the 

 cells, not (in about a score of observed cases) in their mode 

 of origin. The micromeres always arose separately by unequal 

 divisions of the macromeres, as in the normal ovum. 



D. Cleavage of the 2/4 Embryo. 



The early cleavage of the 2/4 embryo was observed in only 

 three cases. In one of these, the first division of the isolated 

 pair of cells was equal (Fig. 85). In the other two it was 

 unequal (Fig. 84), and the ensuing 8-celled stage (Fig. %6) 

 was similar to the unequal Y^ embryos (Fig. 104). The later 

 history was not followed. Several other isolated 2/4 embryos 

 gave rise to normally formed half-sized gastrulas. 



E. Unilateral Development. 



It occasionally happens that only one of the blastomeres of 

 the 2-celled stage develops, as Driesch has likewise observed 

 in Echinus {cf Roux and Chabry). I have obtained only a 

 few such embryos, three of which are figured. Fig. 59 repre- 



