120 Part III. — Sixteenth Annual Heport 



nucleoli lose their metabolic fuuction, so that the secretion derived 

 from the follicular cells is no longer assimilated, and is retained. 



The Histological Changes at Maturation. 



The minute changes occurring in the ovum at maturation, associated 

 with the dissolution of the germinal vesicle, are more difficult to detect, 

 and I have been scarcely more fortunate than other observers in tracing 

 them in the teleostean egg, except, perhaps, in that of the angler, to be 

 presently described. The chromosomes which go to form the polar 

 bodies and the egg nucleus are, of course, very small compared with the 

 germinal vesicle or the nucleoli, and the formation of these structures 

 appears to take place only after fertilisation.* The hyaline enlargement, 

 as has been previously described, appears to take place more rapidly in 

 pelagic than in demersal eggs, and none of my sections show with 

 certainty the alterations in the germinal vesicle. But I have found 

 some transitional stages in the fresh ovaries. In that of the red gurnard 

 I found a translucent egg measuring l"41mm., the fully distended ovum 

 measuring l-55mm., and the opaque about 1mm. By careful illumination 

 the faint outlines of a few enlarged spheres could be made out in the 

 interior of the translucent vitellus. In the ovaries of a flounder which 

 was practically spent, the cavities being collapsed and the contents con- 

 sisting mostly of a number of translucent eggs and abundant fluid, I 

 found some of the eggs presenting transitional appearances. Many of 

 the eggs showed distinct post-mortem changes — that is to say, they 

 exhibited a faint opacity, and on microscopic examination the periblast 

 was found to have shrunk from the zona, to be greyish or brownish, 

 granular and friable. Among these eggs others were found in the 

 earlier stages of expansion. They measured from about 0*72mm. to 

 0*78mm., the fully mature ovum measuring in this species about 

 0'92mm., and the position of the germinal vesicle was defined as a 

 clear spot measuring about 0*22mm. In an isolated germinal vesicle 

 of this size no nucleoli were detected, and it was seen to be very 

 finely granular (Zeiss E. oc. 2). The yolk spheres had increased 

 in size, some of them measuring 0"19mm., and some of them 

 presented the peculiar appearances shown in Fig. 19, PI. I. ; in some, part 

 of the margin was fretted and ragged, in others the whole edge was in 

 this condition, and they had, as a rule, fine granules dispersed through- 

 out them, but some were clear vacuoles. A number of the larger opaque 

 eggs were soaked all night in a 1 per cent, solution of common salt, and 

 in the morning some were found to be enlarged, and showing a clear 

 peri vitelline space. On slight pressure under the cover-glass, the yolk- 

 spheres ran together, forming curious figures ; they were obviously 

 droplets of liquid. The germinal vesicle was small, and measured in one 

 case 0*1 6mm. 



In other eggs in this ovary a lenticular blastodisc was formed which 

 sometimes showed irregular pseudo-cleavage and disintegration, some- 

 times it appeared healthy ; other eggs showed the periblast in continuous 

 contact with the zona. But many full-sized translucent eggs exhibited 

 other features associated with the presence of vacuoles in their interior, 

 some of which are represented in Figs. 20-24, PI. I. The one most closely 

 resembling those above described measured 0'86mm., and in it the 



* Agassiz and Whitman, Mem. Mus. Comp. Zool, Harvard Coll., Vol. xiv., Part ii. 1, 

 p. 12. 1889. Some observers have described the elimination of polar bodies from 

 unfertilised teleostean eggs, as Hoffmann, Cunningham, Kingsley and Cohn ; the latter 

 admit the possibility of the eggs having been fertilised. 



