THE OVUM 97 



spermatozoon touches it. 1 In other forms (insects, birds) the vitelline 

 membrane may be present before fertilization, and in such cases the 

 egg is often surrounded by a chorion as well. The latter is usually 

 very thick and firm and may have a shell-like consistency, its surface 

 sometimes^showing various peculiar markings, prominences, or sculpt- 

 ured patterns characteristic of the species (insects). 2 



The accessory envelopes are too varied to be more than touched 

 upon here. They include not only the products of the oviduct or 

 uterus, such as the albumin, shell-membrane, and shell of birds and 

 reptiles, the gelatinous mass investing amphibian ova, the capsules 

 of molluscan ova and the like, but also nutritive fluids and capsules 

 secreted by the external surface of the body, as in leeches and earth- 

 worms. 



When the egg is surrounded by a membrane before fertilization it 

 is often perforated by one or more openings known as micropyles, 

 through which the spermatozoa 

 make their entrance (Figs. 44, 



45). Where there is but one micro- //^ \ W/"~"XX /> b 



pyle, it is usually situated very 

 near the upper or anterior pole 

 (fishes, many insects), but it may 

 be at the opposite pole (some in- 

 sects and mollusks), Or even On Fig. 45. Upper pole of the egg of Argo- 



the side (insects). In many insects nauta - 

 there is a group of half a dozen or 



more micropyles near the Upper shaped micropyle; below the latter lies the 



pole of the egg, and perhaps cor- ^ % e ' ayer f prot - 

 related with this is the fact that 



several spermatozoa enter the egg, though only one is concerned 

 with the actual process of fertilization. 



The plant ovum, which is usually known as the oosphere (Figs. 

 46, 80), shows the same general features as that of animals, being 

 a relatively large, quiescent, rounded cell containing a large nucleus. 

 It never, however, attains the dimensions or the complexity of struct- 

 ure shown in many animal eggs, since it always remains attached to 

 the maternal structures, by which it is provided with food and invested 

 with protective envelopes. It is therefore naked, as a rule, and is 

 not heavily laden with reserve food-matters such as the deutoplasm 

 of animal ova. A vitelline membrane is, however, often formed soon 

 after fertilization, as in echinoderms. The most interesting feature 



1 That the vitelline membrane does not pre-exist seems to be established by the fact that 

 egg-fragments likewise surround themselves with a membrane when fertilized. (HERTWIG). 



2 In some cases, according to Wheeler, the insect-egg has only a chorion, the vitelline 

 membrane being absent. 



H 



