of the Fishery Board for Scotland. 



99 



however, appear quite clear, and may possibly be of an oily nature. If 

 the spherules be treated with dilute acetic acid the contents rapidly 

 flow together into minute vesicles, which coalesce, the granules dissolving 

 and becoming invisible, while the spherule is indicated by an optical ring 

 which unites with other rings in contact with it. A 5 per cent, solution 

 of acetic acid dissolves the contents instantaneously. 



In species with demersal eggs the yolk-spherules are both larger and 

 of somewhat different character, being much more solid and granular. 

 In the eggs of the angler they may reach a size of 0*08mm., and they 

 are markedly granular in character. In spirit specimens under a high 

 power the spherules appear to be full of solid-looking refringent spheres 

 with an irregular margin, which sometimes appear as angular crystalline 

 bodies of high refrangibility. In the egg of the John Dory the larger 

 spherules may measure 0'05 in sections, and they are full of very dark 

 granules in the fresh state and exhibit an irregular margin. In sections 

 the spherules take on a deep stain, and appear as conspicuous aggrega- 

 tions or concretions of smaller spherules, so that they have a mulberry- 

 like appearance (Fig. 11*, PI. III.). This difference in the character of 

 yolk -spherules in pelagic and some demersal eggs is shown in other ways. 

 Thus, the yolk in the latter is, as a whole, much more resistant to 

 pressure, and after hardening it is difficult to cut. 



As the formation of yolk proceeds, the protoplasm diminishes in 

 amount, and ultimately it is represented by a delicate film between the 

 spherules, which collectively looks like a network in sections, stains com- 

 paratively slightly, and is continuous with the cortical layer externally, 

 and with the wall of the germinal vesicle internally. This interstitial 

 protoplasm between the spherules is in much greater abundance in 

 demersal than in pelagic eggs. The largest spherules, as a rule, are 

 situated about midway between the germinal vesicle and the zona 

 radiata — that is to say, in the zone where they are first formed — and it 

 is obvious that each spherule grows by additions to its substance. With 

 a high power a thin double contour may be made out in certain sections, 

 from which the granular contents have retracted (Fig. 4, PI. III.). 



In the course of the examination of my sections I endeavoured 

 to ascertain the presence of a yolk nucleus, or of chromatic sub- 

 stance, in the cytoplasm, such as have been described in the eggs 

 of Teleosteans by many observers. In many young ova examined 

 in the fresh condition I detected occasionally a highly refringent 

 granule in the cytoplasm, having a faintly greenish hue like the 

 nucleoli in the fresh state, but it may have been a particle of 

 another nature. In sections of ovaries of fishes with pelagic eggs the 

 yolk nucleus is not commonly seen, but two conditions were occasionally 

 found in the cytoplasm of eggs in which the yolk had either not begun 

 to form, or where it was represented by a narrow peripheral zone. In 

 some cases a minute highly-stained body, not larger than a nucleolus, 

 was present, the protoplasm around it not being vacuolated or differen- 

 tiated. In other cases the condition was different, the yolk nucleus being 

 surrounded by a clear zone, and sometimes irregularly shaped or stellate, 

 the rays passing into the protoplasm, and being apparently continuous 

 with the reticulum. In none of my sections is the vitelline body so 

 common or conspicuous as in those of LopMus (Fig. 12, PI. II.). The 

 origin and function of the vitelline nucleus in the cytoplasm of young 

 eggs are of much interest. Bambeke has described the elimination of 

 chromatic substance from the eggs of Scorpcena, and has discussed the 

 various interpretations that have been assigned to the presence of such 



