580 THE STRUCTURE AND DEVELOPMENT 



tributed but are collected in peculiar little masses or aggrega- 

 tions (PL 25, fig. 21). These resemble the granular masses, 

 figured by His (loc. cit. PI. 4, fig. 33) in the Salmon, and may be 

 compared with the aggregations figured by Gotte in his mono- 

 graph on Bombinator igneus (PI. I, fig. 9). It deserves to be 

 especially noted, that when the yolk spherules are first formed, 

 the peripheral layer of the ovum is entirely free from them, a 

 feature which is however apt to be lost in ova hardened in picric 

 acid (PI. 25, fig. 21). Two points about the spherules appear 

 clearly to point to their being developed in the protoplasm of 

 the ovum, and not in the follicular epithelium, (i) 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 mate- 

 rial, 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 egg. 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, 

 according to which the spheres at first present gradually grow 

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

 (nuclei of His ?) appear and form the permanent 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 growth 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 



1 The peculiar oval, or at times slightly rectangular and striated yolk spherules of 

 Elasmobranchs are mentioned by Leydig and Gegenbaur (PI. n, fig. 20), and myself, 

 Preliminary Account of Development of Elasmobranch Fis/us, and by Filippi and His 

 in Osseous Fishes. 



