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 Q.^g 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 



