THE EGG AND ITS MEMBRANES. 47 



THE GERMINAL VESICLE. 



This is eccentric in eggs even as small as 3 mm. {cf. fig. 30). In the section of a 

 well-grown ovarian egg shown in fig. 33 it lies close to the side of the egg. The 

 spireme has here contracted into a minute mass and has given rise to (about) twelve 

 pairs of chromosomes.* These are of remarkably small size, smaller by about 

 one-half than those of a corresponding stage of shark (Pristiurus); and they are 

 also smaller in terms of the germinative vesicle. In Pristiuris (Riickert) the mass of 

 chromosomes at this stage measures 36 /< in width and the vesicle 296 ; in Chimsera 



39 40 



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'j^^ 



41 





iV ' * ■' ' ' '''' '^&^-- '". 



V • * • s^^- 



«■;■'■;■■■-■■ 

 *:v. . 'V, -. 





43 



42 b 





" ^ f^j'-'- ■■-■■-■'■■■ 





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Fig. 39.— Detail of sperm nucleus (rom section of late fertilization stage. The sperm head is surrounded by a conspicuous aster, 

 in some of whose dendritic rays appear the nodes referred to below. The sperm nucleus itself is undergoing amitotic 



division. >.' 473. 

 Fig. 40.- Detail of section of late ferlilizarion stage. Throughout the germinal yolk occur asters which have no apparent 



reference to nuclear structures. At / many of these asters appear around a large granule of yolk. It will be seen that 



the rays are formed as lines in the thickened walls of alveoles. 

 Fig. 41.— Detail of section of late fertilization stage, showing asters in germmal yolk. 

 Fig. 42.— Detail of section of late fertilization stage. At n the egg-nucleus is shown surrounded by a number of asters. 



The asters appear to lack centrosomes and centrosphere. Note as before rays formed from rims of alveoles. 

 Fig. 43.— Section similar to the foregoing. A sperm nucleus, however {>/), is shown surrounded by asters. 



the same mass measures 16 and the vesicle 570. In other words, with a germinal 

 vesicle twice the size, the size of the chromatin mass in Chimaera is but one-half 

 that of the shark. In the shark the chromatin mass measures about one-ninth the 

 diameter of the vesicle; in Chima^ra, on the other hand, about one thirty-eighth. 

 This condition indicates again the greater specialization in the egg of Chimsra. 

 The chromosomes themselves, it will be remarked (figs. 33 c, 33 d), vary considerably 

 in length ; thus the pair shown at x are apparently longer than those at y and at 3, 

 and a detailed examination has convinced the writer that this difference is a real 

 one, /, c, not due to the oblique position of the objects. This observation may be 

 mentioned, since it afiords an additional suggestion as to the individuality of the 

 chromosomes, recently discussed, c\ g. , by Sutton, Wilson, and Moenkhaus. 



*Preliminary to first polar division. The number of chromosomes is clearly much smaller than in sharks (36 in 

 Pristiurus and Torpedo). 



