THE OVUM 



uous nucleolus known to the earlier observers as the germinal spot. 

 In many eggs the latter is single, but in other forms many nucleoli 

 are present, and they are sometimes of more than one kind, as in 

 tissue-cells': 1 In its very early stages the ovum contains a centro- 

 some, but this afterwards disappears from view, and as a rule cannot 

 be discovered until the final stages of maturation (at or near the time 

 of fertilization). It 

 is then found to lie 

 just outside the ger- 

 minal vesicle on the 

 side nearest the egg- 

 periphery where the 

 polar bodies are sub- 

 sequently formed 

 After extrusion of the 

 polar bodies (p. 131) 

 the egg-centrosome 

 as a rule degenerates 

 and disappears. The 

 egg thus loses the 

 power of division 

 which is afterwards 

 restored during fer- 

 tilization through the 

 introduction of a new 



Toxopneustes 



centrosome by the 

 spermatozoon. In 

 parthenogenetic eggs, 

 on the other hand, 

 the egg-centrosome 



Fig. 42. Ovarian egg of the sea-urchin 

 (X75o). 



g.v. Nucleus or germinal vesicle, containing an irregular dis- 

 continuous network of chromatin ; g.s. nucleolus or germinal 

 spot, intensely stained with hsematoxylin. The naked cell-body 

 consists of a very regular network, the threads of which appear 

 as irregular rows of minute granules or microsomes. Below, 

 at s, is an entire spermatozoon shown at the same enlargement 

 (both middle-piece and flagellum are slightly exaggerated in 

 size). 



persists, and the egg 

 accordingly retains the power of division without fertilization. The 

 disappearance of the egg-centrosome would, therefore, seem to be 

 in some manner a provision to necessitate fertilization and thus to 

 guard against parthenogenesis. 



The egg-cytoplasm almost always contains a certain amount of 

 nutritive matter, the yolk or deutoplasm, in the form of solid spheres 

 or other bodies suspended in the meshes of the reticulum and vary- 

 ing greatly in different cases in respect to amount, distribution, form, 

 and chemical composition. 



1 Hacker ('95, p. 249) has called attention to the fact that the nucleolus is as a rule 

 single in small eggs containing relatively little deutoplasm (coelenterates, echinoderms, 

 many annelids, and some copepods), while it is multiple in large eggs heavily laden with 

 deutoplasm (lower vertebrates, insects, many Crustacea). 



