104 



Part ill. — Sixteenth Annual Report 



cytoplasm, which at first consists largely of chromatic particles. There 

 appears, iu short, to be an exodus of chromatic granules into the peri- 

 nuclear zone, and the function of this elimination of chromatin must 

 surely be in connection with the formation of yolk. The primitive ovum 

 has two chief functions — to preserve the chromatin of heredity, aud to 

 produce the cytoplasm, in which the yolk is the chief constituent. The 

 amount of chromatic substance present in the germinal vesicle is far 

 greater than what is required for the egg-nucleus and the polar bodies. 

 The eliminated chromatin does not come from the nucleoli, unless, 

 possibly, but not probably, in the primitive eggs. I have seen nothing to 

 indicate a " budding" from nucleoli, or from the germinal vesicle, as 

 some observers have described ; nor anything resembling a fusion or divi- 

 sion of nucleoli. The nucleoli seem to grow as independent bodies from 

 chromatic *' granules"; whether or not also by fusion of minute granules, 

 I cannot say. At the close of the growth of the ovum we shall see that 

 they break up into a large number of chromatic granules. 



It seems to me that, as the egg grows in size, the manufacture of 

 chromatin is chiefly restricted to a certain part of the reticulum, at or 

 near the centre of the germinal vesicle, namely, at the loops described, 

 and that the particles formed move outwards, some probably to form 

 nucleoli, others to pass into the cytoplasm. At the close of ovarian 

 growth the chromosome loops or spireme is very much smaller, and not 

 more highly stained (Fig. 7, Plate III.). 



The Large Opaque Eggs. 



I now proceed to consider the large opaque eggs shortly before they 

 undergo the translucent enlargement at maturation and acquire the 

 peculiar characters that distinguish the ripe pelagic ovum. Large 

 numbers belonging to various species were examined in the fresh state, 

 and also in stained and mounted sections. The egg-membrane, or so- 

 called zona radiata, when freed from its follicular investment, presents 

 much the same character in them all, consisting of a clear translucent 

 layer that varies in thickness in different species. In demersal eggs it 

 is generally thick and tough, usually of two or more layers, and it may 

 possess tubercles and other processes on its external surface. It is very 

 thin in the flounder and dab, and much thicker in the plaice and cod. 

 Up to the period of maturation the egg-membrane appears to increase in 

 thickness with the growth of the egg, but measurements in mounted 

 sections cannot always be trusted owing to the fact that re-agents act 

 upon it, and may alter it considerably. The addition of a 5 per cent, 

 solution of acetic acid swells up the egg -membrane instantaneously, and 

 softens it so much that the contents break through ; a strong solution of 

 common salt acts in the same way, but much more slowly. The mem- 

 brane of plaice eggs may swell to a thickness of 0*2mm. — many times 

 more than the normal thickness — in a short time. 



The egg-membrane may present difl'erences in markings as well as 

 differences of thickness in various species. The markings are usually of 

 two kinds, both of which are generally found together. One consists in 

 the very common presence of wrinkles or creases, which exhibit anasto- 

 mosing or convoluted irregular ramifications on surface view, and can be 

 shown by careful focussing to be elevations or depressions in the 

 substance. The second condition consists of very minute dotting or 

 punctation, which may be arranged in more or less radial lines. In the 

 mature egg of the plaice I have found this dotted pattern to be limited 

 to an extremely thin superficial pellicle, which may spontaneously 



