OF THE VERTEBRATE OVARY. 579 



the late stages, when the yolk spherules are fully developed, it 

 is difficult to observe this network, but, as has been shewn in my 

 monograph above quoted, it is still present after the commence- 

 ment of embryonic development. An arrangement of the proto- 

 plasmic striae like that described by Schultz has not come" under 

 my notice. 



The development of the yolk appears to me to present spe- 

 cial difficulties, owing to the fact pointed out by His 1 that the 

 conditions of development vary greatly according to whether 

 the ovary is in a state of repose or of active development. I do 

 not feel satisfied with my results on this subject, but believe 

 there is still much to be made out. Observations on the yolk 

 spherules may be made either in living ova, in ova hardened in 

 osmic acid, or in ova hardened in picric or chromic acids. The 

 two latter reagents, as well as alcohol, are however unfavourable 

 for the purpose of this study, since by their action the yolk 

 spherules appear frequently to be broken up and otherwise 

 altered. This has to some extent occurred in PI. 25, fig. 21, and 

 the peculiar appearance of the yolk of this ovum is in part due 

 to the action of the reagent. On the whole I have found osmic 

 acid the most suitable reagent for the study of the yolk, since 

 without breaking up the developing spherules, it stains them 

 of a deep black colour. The yolk spherules commence to be 

 formed in ova, of not more than 0*06 mm. in the ovaries of 

 moderately old females. In young females they are apparently 

 not formed in such small ova. They arise as extremely minute, 

 highly refracting particles, in a stratum of protoplasm some little 

 way bclou 1 the surface, and are always most numerous at the pole 

 opposite the germinal vesicle. Their general arrangement is very 

 much that figured and described by Allen Thomson in Gaster- 

 osteus 2 , and by Gegenbaur and Eimer in young Reptilian ova. 

 In section they naturally appear as a ring, their general mode of 

 distribution being fairly typically represented on PI. 25, fig. 27. 

 The ovum represented in fig. 27 was o - 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 developing yolk spherules are not uniformly dis- 



1 Das Ei bd Knochenfischen. 



z "Ovum" in Todd's Encyclopedia, fig. 69. 



