THE OVUM 131 



(Fig. 58), which have little 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 egg (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 Biitschli. 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 rhomboidal 

 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 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 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 single 

 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 egg 

 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 



