STRUCTURE AND DEVELOPMENT OF VERTEBKATE OVARY. 409 



yolk, and closely agree with what Gegenhaur has given in the 

 paper above quoted more fully for Aves and lieptilia than for 

 Elasmobranchii. 



In very young ova the body of ovum is simply granular, 

 but when it has reached about U'5 mm. tlie granules are seen to 

 be arranged in a kind of network, or spongework (PI. XVIII, 

 fig 21), already spoken of in my monograph on Elasmobranch 

 Eishes. 



This network becomes more distinct in succeeding stages, 

 especially in chromic acid specimens (PL XVIII, fig. 22), probably 

 in part owing to a granular precipitation of the protoplasm. In 

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

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

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

 ment of embryonic development. An arrangement of the proto- 

 plasmic strise like that described by Schultz has not come under 

 my notice. 



The development of the yolk appears to me to present special 

 difficulties, owing to the fact pointed out by His^ that the con- 

 ditions 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 unfavorable 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. XVIII, fig. 21, and the pecu- 

 liar 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 heloio the sur- 

 face, and are always most numerous at the pole opposite the ger- 

 minal vesicle. Their general arrangement is very much that figured 

 and described by Allen Thomson in Gasterosteus,^ and by Gegen- 

 baur 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. XIX, fig. 27. The ovum represented 

 ' ' Das Eie bei Knochenfischen.' 

 * " Ovum," iu ' Todd's Cyclopaedia,' fig. 69. 



