410 F, M. BALFOUR. 



in fig. 2T, was 0*5 mm. in diameter, and the yolk spherules were 

 already largely developed ; in smaller ova they are far less 

 numerous, though arranged in a similar fashion. The develop- 

 ing yolk spherules are not uniformly distributed but are collected 

 in peculiar little masses or aggregations (PL XVIII, fig. 21). 

 These resemble the granular masses, figured by His (loc. cit. 

 PI. IV, fig. 33) in the Salmon, and may be compared with the 

 aggregations figured by Gotte in his monograph on Bombinator 

 igneus (Plate I, fig. 9). It deserves to be especially noted, that 

 when the yolk spherules are first formed, Xht peripheral layer of 

 the ovum is entirely free from them, a feature which is howevei 

 apt to be lost in ova hardened in picric acid (PI. XVIII, fig. 21). 

 Two points about the spherules appear clearl}'^ to point to their 

 being developed in the protoplasm of the ovum, and not in the 

 follicular epithelium. (1) That they do not make their appearance 

 in the superficial stratum of the ovum. (2) That no yolk spherules 

 are present in the cells of the follicular epithelium, in which they 

 could not fail to be detected, owing to the deep colour they assume 

 on being treated with osmic acid. 



It need scarcely be said that the yolk spherules at this stage 

 are not cells, and have indeed no resemblance to cells. They 

 would probably be regarded by His as spherules of fatty material, 

 unrelated to the true food yolk. 



As the ova become larger the granules of the peripheral 

 layer before mentioned gradually assume the character of the 

 yolk spheres of the adult, and at the same time spread towards 

 the centre of the i^u,^. Not having worked at fresh specimens, 

 I cannot give a full account of the growth of the spherules ; but 

 am of opinion that Gegenbaur's account is probably correct, ac- 

 cording to which the spheres at first present gradually grow 

 and develop into vesicles, in the interior of which solid bodies 

 (nuclei of llis ?) appear and form the ])ermanent yolk spheres. 

 When the yolk spheres are still very small they have the typical 

 oblong form^ of the ripe ovum, and this form is acquired while 

 the centre of the ovum is still free from them. 



The jjrowth of the yolk appears mainly due to the increase in 

 size and number of the individual yolk spheres. Even when the 

 ovum is quite filled with large yolk spheres, the granular proto- 

 plastic network of the earlier stages is still present, and serves to 

 Jiold together the constituents of the yolk. In the cortical 

 layer of nearly ripe ova, the yolk has a somewhat different cha- 

 racter to that which it exhibits in the deeper layers, chiefly 



* The peculiar oval, or at times sli^lilly rcclanijular and striated yolk 

 spherules of Elasmobranchs are nieutioiud l)y Leydig and Gegciibaur (pi. xi, 

 fig. 20), and tnysclt", ' L'rcliniinary Account of J)cvc!opmcut of Ehismobraucll 

 I'islics,' and by i''ilii)iu and liis iu ' Osseous Fishes.' 



