110 



Part III. — Sijjteenth Anmuil Report 



layer may be demonstrated in the fresh opaque egg by careful examina- 

 tion of the periphery of the vitellus, especially when it is retracted from 

 the zona radiata (Fig. 10, PI. I.). It is also well seen in mounted and 

 stained sections (Fig. 7, PI. I.) as a thin stained layer within the zona. 

 It is the only part of the cytoplasm of the pelagic ovum which retains its 

 position and appearance after the transformation at maturation, when, 

 supplemented by the interstitial protoplasm, it forms the periblast. It 

 can be better studied in fresh eggs than in sections, with respect, at least, 

 to the numerous very minute, bright, shining granules imbedded in it, 

 and which are optically indistinguishable from minute yolk-spherules. 

 The protoplasmic pellicle is itself very thin, measuring in the large 

 opaque ovum of the flounder not more than about 0 015mm., and the 

 granules are crowded together much more closely than in the mature 

 ovum, in which, after fertilisation, they become aggregated in clusters 

 or strings. In the mature eggs of the grey gurnard they measured 

 0*005mm. They are sub-equal in size and fairly regularly scattered in 

 the protoplasmic pellicle of the mature unfertilised egg, from ten to 

 eighteen being found within a square with a side of O'lOmm. Kupffer 

 called these bodies in the egg of the herring yolk-granules (Dotter- 

 korner). Ryder* and Aga.?siz and Whitman! applied no special name to 

 them, and their signiticancB and fate are not very clear. They resist dilute 

 acetic acid, and according to Ryder they also resist the action of ether 

 and alcohol, and they do not stain. In mature eggs which have been 

 dead for days they persist unchanged, even after the periblast has greatly 

 shrunken and is partially disintegrated. They are also interesting from 

 the circumstance that, apart from oil-globules when these are present, 

 they are the only definite bodies which can be recognised to remain 

 unchanged within the ovum during the final stage of maturation. The 

 yolk-spherules, germinal vesicle, and nucleoli all vanish, but thb cortical 

 layer with its granules persists. 



The large opaque pelagic eggs, then, before they begin to undergo the 

 physico-chemical changes at maturation, and while still invested by the 

 follicular epithelium, consist of the following parts : — The egg-membrane 

 or zona radiata, which is (relatively to the mature egg) thick ; an 

 extremely delicate membrane within, at least in some cases, which covers, 

 and follows in its retraction, the vitellus ; a thin cortical layer of proto- 

 plasm with minute shining granules imbedded in it, which is directly 

 continuous with, and may be regarded as a peripheral enlargement of, the 

 protoplasmic raeshwork between the yolk spherules and granules ; an 

 enormous number of yolk-spherules of very various sizes which compose 

 the great mass of the egg; in many cases oil-globules which are situated 

 around the germinal vesicle ; a germinal vesicle of large size, centrally 

 placed, possessing a distinct soft membrane, with which the protoplasmic 

 matrix of the cytoplasm is continuous, and containing numerous more or 

 less spherical nucleoli. 



The ^^ATURE Egg and the Changes at Maturation. 



The mature pelagic ovum, as previously stated, has a very different 

 appearance from the opaque eggs of the preceding stage, and its structure, 

 so far as it may be distinguished, is much more simple. In consists of 

 the envelopes described, which are, however, much stretched ; the thin 

 cortical layer of germinal protoplasm, with its shining granules, which 



* U.S. Fish Commis. Report fw 1882, p. 45:.. 



t Proc. AiMT. Acad. Arti and Sci.. xii., p. 23. 1885. 



