THE OVUM 



1^7 



early observers. There are some cases {e.g. echinoderm eggs) in 

 which it is always a single large spherical body (Fig. 58), and this 

 condition appears to be characteristic of the very young ovarian e^'-f^s 

 of most animals. As a rule, however, the number of nucleoli ^in- 

 creases with the growth of the ovum, until, in such forms as Amphibia 

 and reptiles, they may be numbered by hundreds. 



In a large number of cases the nucleoli are of two quite distinct 

 types, which Flemming has distinguished as the *' principal nucleolus " 



Fig. 59. — Ovum of the cat, within the ovary, directly reproduced from a photograph of a 

 preparation by Dahlgren. [Enlarged 235 diameters.] The ovum lies in the Graafian follicle 

 within the discus proligerus, the latter forming the immediate follicular investment {corona 

 radiata) of the Qg^. Within the torona is the clear zona pellucida or egg-membrane. {Cf. 

 Fig. 92.) 



{Hajiptmicleolus) and ''accessory nucleoli" {Ncbennuclcoli). These 

 differ widely in staining-reaction ; but it does not yet clearly appear 

 whether they definitely correspond to the plasmosomes and karyo- 

 somes of tissue-cells (p. 34). The principal nucleolus, which alone 

 is present in such eggs as those of echinoderms, often stains deeply 

 with chromatin-stains, yet differs more or less widely from the 

 chromatin-network,! and in some cases at least it does not contribute 



1 Cf. List, '96, Montgomery, '98, 2, and Obst., '99. 



