PRIMORDIAL EGG-CLEAVAGE. 



55 



other hand, compound organisms or individuals of higher 

 order — social aggregations of a number of cells. The 

 earliest of these, which, under the name of Synamoebse, 

 we must rank as the third stag^ of our pedigree, are quite 

 simple societies of all homogeneous undifferentiated cells ; 

 amoeboid communities. To be certain as to their nature 

 and origin, we need only trace the ontogenetic product of 

 the parent-cell step by step. After the cytula (Fig. 166) 

 has originated, by the re-formation of a cell-kernel, from 



Fig. 169. — Original or primordial egg-cleavage. The parent-cell, or 

 cytula, which resulted from the fertilization of the egg-cell, first breaks up, 

 by a continuous and regular process of dirisioii, into two cells (A), then into 

 four (B), then into eight (C), and, lastly^ iato very numerous cleavage- 

 cells (D). 



the Morula (Fig. 165), the parent-cell breaks up, by repeated 

 division, into numerous cells. We have already minutely 

 examined this important process of 

 egg-cleavage, and have found that all 

 the various modes of the latter are 

 modifications of a single mode, that 

 of original or primordial cleavage. 

 (Cf. Chap. YIIL, p. 188.) In the Ver- 

 tebrate line this palingenetic form of 

 egg-cleavage has been accurately re- Fig. 170.-Muibeny. 



°° ° •^ germ, or morula. 



