HOW THE CYTOPLASM ACTS 



Fig. 48. — Cleavage, blastula and gastrula stages in the development of the egg of 

 the thread worm Ascaris megaloccphala (diploid race with one gametic chromosome). 



A, the maternal and paternal chromosomes meet on the first cleavage spindle. 



B, formation of two cells, ditferentiated in cytoplasmic determinants, the dorsal 

 with the potentiality for more rapid division, leading to fragmentation of its 

 chromosomes at anaphase and loss of the distal heterochromatin, the ventral with 

 the potentiality for regular mitosis without breakage. Cj and C._, views of the second 

 divisions: the differentiation continues with progeny of the cell which divides in 

 the same axis as that of the first mitosis. D, the T-shaped embryo rounded off to 

 form the blastula. E, F, third cleavage division. G, the gastrula stage: 32 cells of 

 which one only has the complete set of chromosomes and will give rise to the 

 germ cells. The cytoplasmic germ-line determinants, wliich are directly visible 

 in copepods and are here marked only by the presence of fewer vacuoles are shown 

 here by stippling (cf. Bounure, 1939). 



Note. — The difference in mitotic rates suggests a gradient in nucleic acid content 

 which should be exaggerated by the absorption of lost heterochromatin in the 

 cytoplasm (after Boveri's figures 1910 et al., but conflicting with the diagrams in 

 Wihon, 1900; Bovcri, 1904; Morgan, 1913; and Belar, 1928). 



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