THE EGG AND ITS MEMBRANES. 



43 



fig 3 1 ; here it will be seen that the tunic, although syncytial in character, is clearly 

 divided according to its nuclei into three layers outmost, middle, and inmost. In 

 the outmost layer the nuclei are small and closely compressed oblong, therefore, 

 in form and directed ecto-entad ; in the middle layer they are large and diffuse ; 

 in the inmost, small, irregular, and closely apposed to the basement membrane. 

 Between the outmost layer of the tunic, o, and the gonadial sinus, gs, the tissue of 

 the ovary contains numerous strands of connective tissue interspersed with plasma 

 spaces, c; it is from these, doubtless, that nutriment is passed through the special- 

 ized tunic to the inclosed egg. During this process it may be assumed that the 

 various types of nuclei of the tunic play definite parts ; thus the closely compacted 

 nuclei of the outmost layer purvey nutriment from the plasma spaces (and capil- 

 laries) to the dilated elements of the middle layer. These again transfer their 

 nutriment to the small nuclei which are closely apposed to the egg. 



.33* 



Fig. 33. Section of region of germinative vesicle of well-grown ovarian egg (about 20 mm. in diameter), gy. Germinal 

 yolk; gs, gonadial sinus; gv, large germinal vesicle containing near its center a group of chromosomes; mgv, limiting 

 membrane of germinal vesicle ; t, tunic ; zr, zona radiata. 



Fig. 33A. Detail of preceding section from point at side of egg, showing absence in this region of the zona radiata. In this 

 region yolk granules are developed in numerous minute vacuoles. 



Fig. 33B. Detail of section of fig. 33, taken at a point where the side of the germinal vesicle recedes from surface of egg. 

 gy, Germinative yolk ; mgi', membrane forming wall of germinal vesicle ; /, tunic, showing large vacuoles ; z, zona 

 radiata. This will be seen to extend only over the margin of the vesicle. Between the vesicle and the adjacent tunic 

 there extends only a thin peripheral layer of germinative yolk. 



Figs. 33C and 33D. Detail of chromosomes shown in fig. 33 (two sections). X 385. 



The differences in the nuclei of the tunics are apparently physiological, since 

 intergrading forms occur ; thus in the figure cited a nucleus of the inmost layer 

 is clearly connected with a nucleus of the middle layer. In some cases such a rela- 

 tionship is demonstrated by dividing nuclei, which, it may be remarked, exhibit 

 sometimes direct, sometimes indirect division. At the surface of the egg is a sharply 

 marked membrana limitans ; below this, irregular in thickness, a zona radiata, prob- 

 ably homologous with the well-known layer in eggs of other fishes. Below this the 

 egg shows an outer finer layer and an inner coarser or reticular layer, in which 

 large vacuoles frequently occur. At a somewhat later stage (egg measuring about 



