PRIIIOKDIAL EGG-CLEAVAGE. 



55 



other liand, comp(.>iind organisms or individuals of higher 

 order — social aggregations of a number of cells. The 

 earliest of these, which, under the name of Synamceba?, 

 ve must i-ank as the third stag-e of our pedigree, are quite 

 simple societies of all homogeneous imdifferentiated cells ; 

 amceboid communities. To be certain as to their nature 

 and origin, we need onljr trace the ontogenetic pi'oduct of 

 the iiarent-coll step by step. After the cytula (Fig. 106) 

 has originated, by the re-fc;irmation ijf a cell-kernel, from 



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

 cytnla, whicli resulted from the fertilization of the egg-cell, first breaks np, 

 by a continuous and regular process of division, into tvro cells (.4), then iuto 

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

 cells (D). 



the Morula (Fig, 1(5.5), the parent-cell breaks up, 1)y repeated 

 division, into numerous cells. We have aJrcadv minutely 

 examined this important process of 

 egg-cleavage, and have found tliat all 

 the vaiious modes of the latter are 

 modifications of a single mode, that 

 of original or |)iimorilial cleavage. 

 (Cf. Chap. VIII., p. 18,SJ In the Ver- 

 tebrate line this palingenetic form of 

 egg-cleavage has Ijeen accurately re- 



20 



Fig. 170.— llulberr.F- 

 germ, or morula. 



