HISTOGENESTS 



27 



close spireme, resting daughter nucleus with its daughter centrosome 

 and ultimately a nucleolus (Figs. :><>, k, 1, in, n). Where a cell plate 

 appears, division is consummated without constriction. In certain 

 pathological tissues, e.g., cancers, the cells divide in various 

 atvpical ways, involving the formation of tri- and multipolar spin- 

 dles. 



HISTOGENESIS 



Every higher organism begins as a fertilized egg or zygote; this in- 

 volves the fusion of a male (spermatozoon) and a female (egg) germ cell 

 (Fig. 31, a and e). The result of the fusion is a mingling of approxi- 

 mately equal parts of 

 paternal and mater- 

 nal chromatin (pre- 

 sumably the basis of 

 specific heredity) ; a 

 large mass of mater- 

 nal cytoplasm and 

 nutritive substance 

 with a small, but 

 perhaps important 

 mass of male cyto- 

 plasm; and a coinci- 

 dent stimulus to de- 

 velopment. The fer- 

 tilized egg divides by 

 mitosis into two 

 spheroidal cells the 

 typical embryonic 

 form orblastomeres 

 (Fig. 31, f and g), 

 and each of these 



FIG. 32. TRANSVERSE SECTION OF A FROG EMBRYO, 

 SHOWING THE THREE GERM LAYERS. 



a, neural crest; b, neural groove; c, neural plate; d> 

 ccelom; e, ectoderm;/, mesoderm; g, entoderm; h, somite; 

 i, notochord; j, parietal niesoderm; k, visceral meso- 

 derm; I, yolk; central opening, the primitive intestine. 

 (Drawing by G. A. Pagenstecker.) 



again into two, the 

 segmentation process 

 continuing until the adult organism results as an aggregation of innumer- 

 able cells. This process of growth through cell multiplication is accom- 

 panied by cell differentiation, which constitutes histo genesis. The first 

 outstanding stage in the differentiation is that when the three funda- 



