94 THE GERM-CELLS 



Chapter V. that in some cases a large part of the chromatic reticulum 

 is cast out, and degenerates at the time the polar bodies are formed. 

 It would seem that the nucleoli may likewise represent a portion 

 of the unused chromatin, more closely aggregated and more or less 

 modified in a chemical sense. 



2. The Cytoplasm 



The egg-cytoplasm varies greatly in appearance with the varia- 

 tions of the deutoplasm. In such eggs as those of the echino- 

 derm (Fig. 42), which have little or no deutoplasm, the cytoplasm 

 forms a regular reticulum, which is perhaps to be interpreted as 

 an alveolar structure. Its meshes consist of closely set intensely 

 staining granules or microsomes embedded in a clearer ground- 

 substance. The latter, which fills the spaces of the network, is 

 apparently homogeneous, and contrasts sharply with the micro- 

 somes in staining capacity. In eggs containing yolk the deutoplasm- 

 spheres or granules are laid down between the meshes of the net- 

 work ; and if they are very abundant the latter may be very greatly 

 reduced, the cytoplasm assuming a pseudo-alveolar structure (Fig. 43), 

 much as in plant-cells laden with reserve starch. In many cases 

 a peripheral layer of the ovum, known as the cortical or peri- 

 vitelline layer, is free from deutoplasm-spheres, though it is continu- 

 ous with the protoplasmic network in which the latter lie (Fig. 43). 

 Upon fertilization, or sometimes before, this layer may disappear by a 

 peripheral movement of the yolk, as appears to be the case in Nereis. 

 In other cases the peri-vitelline substance rapidly flows towards the 

 point at which the spermatozoon enters, where a protoplasmic germi- 

 nal disc is then formed ; for example, in many fish-eggs. 



The character of the yolk varies so widely that it can here be con- 

 sidered only in very general terms. The deutoplasm-bodies are com- 

 monly spherical, but often show a more or less distinctly rhomboidal 

 or crystalloid form as in amphibia and many fishes, and in such cases 

 they may sometimes be split up into parallel lamellae known as yolk- 

 plates. Their chemical composition varies widely, judging by the 

 staining-reactions ; but we have very little definite knowledge on this 

 subject, and have to rely mainly on the results of analysis of the total 

 yolk, which in the hen's egg is thus shown to consist largely of pro- 

 teids, nucleo-albumins, and a variety of related substances which are 

 often associated, with fatty substances and small quantities of car- 

 bohydrates (glucose, etc.). In some cases the deutoplasm-spheres 

 stain intensely with nuclear dyes, such as haematoxylin ; e.g. in many 

 worms and mollusks ; in other cases they show a greater affinity for 

 plasma-stains, as in many fishes and amphibia and in the annelid 



