REPROD UCTION. 



907 



There results from fertilization, it is perceived, a single cell complete in all 

 its essential parts. This is the starting-point of the new individual. A pause 

 or resting period usually follows fertilization, and then growth begins. 



Segmentation. The process of growth is a complex process of repeated 

 cell-division, increase in bulk, morphological differentiation, and physiological 

 division of labor. 



Cell-division is largely, if not wholly, indirect or karyokinetic. The term 

 segmentation, or cleavage, of the ovum is conveniently applied to the first few 



FIG. 313. Stages in the segmentation of the egg (continued from Fig. 312). 



D. The divergence of the chromosomes has ceased and the latter have become converted into vesicu- 

 lar masses beside the centrosomes. The spindle is becoming resolved into ordinary cytoplasm. The 

 division of the cytoplasm is beginning with a constriction at the surface of the egg. 



E. The vesicular chromatic masses have become converted into two typical resting nuclei, each with 

 a chromatic network. The single aster, formerly connected with each nuclear mass, has become divided 

 into two, which have taken positions at opposite poles of the nuclei. The division of the cytoplasm is 

 complete, and the two resulting cells, or blastomeres, are resting, preparatory to a second division in a 

 plane at right angles to that of the first. 



divisions, although the details of segmentation are not different fundamen- 

 tally from those manifested later in the division of more specialized cells. 

 Each division may be resolved into three definite acts, which, however, 



