328 GERMINAL ORGANIZATION INDUCTION PHENOMENA 



b 





Fig. 19. Chromosome diminution and RNA distribution in Parascaris eq. (var. bivalens) 

 eggs (fixed in Carnoy, sections stained with Unna-Brachet). (a) Shortly after fertihzation; 

 the ist maturation spindle is in anaphase with 2 pairs of dyads; the incorporated sperma- 

 tozoon is hollowed out by a vesicula still containing the refingent sphere (in grey), the 

 cytoplasm is filled with RNA (darker gray), the nucleus is still compact (black); the egg 

 cytoplasm has only some dispersed RNA granules, (b) During telophasis of the same matu- 

 ration mitosis; more RNA has been formed in the sperm cytoplasm, which disperses 

 progressively; the male nucleus has grown, but still is homogeneous; the RNA of the egg 

 cytoplasm is located in larger granules, (c) Ana-telophase of the 2nd maturation mitosis. The 

 paternal chromosomes disperse in the RNA-rich central area, while the egg RNA granules 

 gather in the surrounding territory, (d) Pronuclei stage, with the 2nd polar body sticking to 

 the egg (below) ; RNA granules are localized in a starlike structure near the pronuclei, (e) 

 Prophase of first cleavage showing the two centrospheres and the two long chromosomes ; 

 each centrosphere contains the same quantity of RNA granules, (f ) Telophase of the first 

 cleavage, the larger upper cell (AB) contains the animal pole ; the smaller, vegetative cell 

 (P) has more RNA granules in its centrosphere. (g) The AB cell has divided, with diminution 

 of its 4 chromosomes (3 visible) ; the P cell, in metaphasis, shows asters congested with RNA, 

 especially in the vegetative one. (h) Further stage of cleavage, with complete chromosomes 

 in the vegetative daughter cell of P, where the asters are again RNA-cok^^jW, especially 

 the vegetative one. (i) The cell protected by the RNA granules against diminution of the chromosomes 

 becomes the initial cell of the germline, and is recognizable by its larger nucleus and RNA- 

 rich cytoplasm. After Pasteels, 1948. 



