THE OVUM 131 



(Fig. 58), which have Httle or no deutoplasm, the cytoplasm forms a 

 regular meshwork, which is in this case an undoubted alveolar struc- 

 ture, the structure of which has already been described at p. 28. In 

 eggs containing yolk the deutoplasm-spheres or granules are laid 

 down in the spaces of the meshwork and appear to correspond to the 

 alveolar spheres of the echinoderm ^^^g (p. 50). If they are of large 

 size the cytoplasm assumes a *' pseudo-alveolar" structure (Fig. 60), 

 much as in plant-cells laden with reserve starch; but reasons have 

 already been given (p. 50) for regarding this as only a modification 

 of the "primary" alveolar structure of Butschli. There is good 

 reason to believe, however, that the egg-cytoplasm may in some cases 

 form a true reticular structure with the yolk-granules lying in its 

 interstices, as many observers have described. In many cases a pe- 

 ripheral layer of the ovum, known as the cortical or peri-vitelline layer, 

 is free from deutoplasm-spheres, though it is continuous with the 

 protoplasmic meshwork in which the latter lie (Fig. 60). 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 

 toward the point at which the spermatozoon enters, where a proto- 

 plasmic germinal 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 rhomb(^idal 

 or crystalloid form as in Amphibia and some 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 ^gg 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 hsematoxylin ; e.i:^. in many 

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

 plasma-stains, as in many fishes and Amphibia and annelids ( Fig. 60). 

 Often associated with the proper deutoplasm-spheres are drops of oil, 

 either scattered through the yolk (Fig. 60) or united to form a .jingle 

 large drop, as in many pelagic fish-eggs. 



The deutoplasm is as a rule heavier than the protoplasm ; and in 

 such cases, if the yolk is accumulated in one hemisphere, the ^gg 

 assumes a constant position with respect to gravity, the egg-axis 

 standing vertically with the animal pole turned upward, as in the 

 frog, the bird, and many other cases. There are, however, many 



