DEVELOPMENTAL EVOLUTION 



231 



The cell, here, represents the most elementary mode of the 

 operation of vital energy, in the conduct of the affairs of life, 

 on the inorganic matter of nature, by which it becomes 

 organic, and subservient to the requirements of metabolism, 

 and capable of assuming, by formative law, the condition, by 

 mitotic division and subdivision, of multicellular organis- 

 ation which forms the second well-marked division of 

 developmental evolution (see Figs. 76, 77). During this 



FIG. 92. FIRST STAGES OF SEGMENTATION OF A MAMMALIAN OVUM ; 

 SEMI-DIAGRAMMATIC. (Drawn by Dr. Allen Thomson, after Ed. v. 

 Beneden's description.) 



z.p., zona pellucida ',p.gl., polar globules ; ect., ectomere ; ent., entomere. a, division 

 into two blastomeres ; b, stage of four blastomeres ; c. eight blastomeres, the 

 ectpmeres partially enclosing the entomeres; d, s, succeeding stages of segmen- 

 tation showing the more rapid division of the ectomeres and the enclosure of 

 the entomeres by them. 



stage, as the process of mitosis, or kariokinesis, advances, 

 the proliferating cells assume the general structural form 

 of blastoderm (Figs. 94, 95), which in turn assumes 

 a, layered, or stratiform, arrangement, the precursor 

 of a structural division, of the evolving embryonic mass. 

 These blastodermic strata consist of the ectoderm, the 

 mesoderm and the hypoderm, which, in continuing the 

 developmental changes, lose their, more or less, parallel 

 cellular arrangement, and assume the character of histo- 

 logically formed structures, in which they ultimately 

 become converted into the embryonic formation, known 

 as the neurenteric canal and adventitial textures. 



