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Part III. — Sixteenth Anrncal Report 



In the angler, while the nearly mature eggs present the usual appearance 

 uf dense opacity to transmitted light and chalky whiteness to reflected 

 light, the almost mature egg looks like a melting snowball or a moistened 

 lump of sugar, and the yolk spherules are seen to be clearer and larger 

 (Fig. 5, PI. II.). The egg is also now much softer and more readily 

 ruptured. In the herring the translucent change is even more marked, 

 and some of the yolk-spherules, in the fresh egg, measure fully 0*09mni. 

 On rupturing the capsule some of them coalesce and flow out as large 

 watery-looking globules. The increase in volume is much less than in 

 the pelagic eggs. A series of measurements of 46 large opaque eggs of 

 the herring gave a mean diameter of l'18mm., as compared with a mean 

 diameter of l'33mm. for 62 of the ripe semi-translucent eggs. The 

 respective volumes were, therefore, about 0-86 cubic millimetres and 1*23 

 cubic millimetres. The difi'erent condition in the two cases is also well 

 brought out in mounted sections, as is shown in Figs. 16, 17, PI. I.; but it 

 is to be noted that in the ripe egg of the herring, as well as of the angler, 

 the process of preparation causes the yolk spheres to coalesce still further, 

 so that they appear much larger than in the fresh eggs, and they also for 

 the most part lose their spherical shape. The commencement of the 

 aggregation of protoplasm to form the large blastodisc is well represented 

 in Fig 17. It would appear from a consideration of the eggs of the 

 herring and angler that the entrance of fluid into the demersal ovum at 

 the close of ovarian life takes place more slowly than in the case of 

 pelagic eggs. I have examined herring roes in which the whole mass of 

 eggs distinctly showed incipient translucency while the germinal vesicle 

 was still present. 



It thus appears that in the case of j', the pelagic egg. Natural Selection 

 has, so to speak, seized upon and greatly exaggerated a phenomenon 

 common to the teleostean egg for another and quite different purpose — 

 namely, to enable the egg to float. 



Certain questions arise in connection with the entrance of watery fluid 

 into the egg: — (1) What is the composition of the fluid? (2) what is its 

 origin'? (3) how is the absorption efi'ected? (4) what determines the 

 change at the close of ovarian growth 1 



It is reasonable to assume that the fluid which enters the egg is 

 secreted by the cells of the follicle, because the egg is everywhere closely 

 invested by these cells, and if it is not secreted by them it must pass 

 through them from the tissues around ; but the free fluid in the 

 stroma is blood plasma, which has itself a much higher specific gravity 

 than the mature egg, and it therefore cannot be the agent. There is no 

 other structure in the ovary, except the follicle, to which the origin of the 

 watery fluid can be assigned. The follicle of the growing pelagic ovum 

 consists of a continuous single layer of minute flattened cells closely 

 applied to the surface of the egg-capsule, and with a small amount of 

 connective tissue around it containing a meshwork of capillary blood- 

 vessels (Fig. 14, PI. I.). It forms a thin membranous tunic of moderate 

 toughness, and may sometimes remain unruptured when the zona is 

 ruptured by pressure, and the yolk then passes between it and the latter. 

 In demersal eggs it is much thicker and more resistant, the granulosa 

 layer being frequently double. The connective tissue stroma of the ovary 

 is very loose ; it also is more abundant and dense in ovaries producing 

 demersal eggs.* The follicular cells of the plaice, for example, are 



* I may say here that one may infer with considerable certainty, from a comparatively 

 early stage in ovarian development, whether the mature eggs are likely to be demersal or 

 pelagic, from the conditions stated, the double-layered or single-layered zona, and the 

 character of the yolk spherules. 



