PRIMOBDIAL 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 Synamcebse, 

 we must rank as the third stage 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 division, into two cells (A), then into 

 four (B), then into eight (C). and, lastly, into 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. VIII., p. 188.) In the Ver- 

 tebrate line this palingenetic form of 

 egg-cleavage has been accurately re- 

 26 



Fig. 170. — Mulbeny- 

 germ, or morula. 



