168 



RALPH S. LILLIE. 



stage. This egg presents an unusually regular appearance ; typi- 

 cally the fusions follow no definite order and from the S-cell stage 

 complexes are more frequently obtained that consist of three, four 

 or five more or less rounded and in part compound blastomeres 

 of unequal size, the entire group remaining enclosed within the 

 egg membrane. Contiguity seems to be the chief condition that 

 determines which of the blastomeres undergo fusion ; there is no 

 indication that sister blastomeres reunite more readily than cells of 

 different parentage ; the fusions take place also after different in- 

 tervals and with varying degrees of completeness. Thus a strict 

 reversal of cleavage in the sense that the egg reverts to the stage 

 immediately preceding (from 16 to 8 cells, etc.) is of exceptional 

 occurrence. It is of interest to note that fusion is typically pre- 

 ceded by the assumption of a perfectly rounded or spherical form 

 by each cell. The flattening at the surfaces of apposition is thus 

 replaced by a curvature indicative of increased surface-tension 

 (Fig. 2). 



FIG. 3. Different stages of fusion 

 of 2-cell stage in m-glycerine solution 

 ( Aster ins ) . 



FIG. 2. Round- 



FIG. I. Example of ing of blastomeres in 

 regular fusion of hlasto- fusion-product of 4- 

 meresof 8- celled stage, celled stage. Urea- 

 Urea solution. (As- solution (Asterias). 

 fen as) . 



In m-glycerine solutions fusions also occur, although in rel- 

 atively fewer instances. Glycerine is specifically injurious to a 

 far higher degree than urea, and swelling and granular disintegra- 

 tion early appear in many eggs. Different stages of fusion of 

 blastomeres of the 2-cell stage are represented in Fig. 3. Fre- 

 quently the separate blastomeres merely become rounded with- 

 out undergoing fusion. 



In m-cane-sugar solutions the tendency towards fusion is even 

 less, in consequence apparently of the extreme viscidity and den- 

 sity of the solution and its tendency to withdraw water from the 



