THE OVUM 



133 



m 



The accessory envelopes are too varied to be more than touched 

 upon here. They inchide 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 ^g% is surrounded by a membrane before fertilization it 

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

 through which the spermatozoa make their entrance (Figs. 62, 61). 

 Where there is but one micropyle, 

 it is usually situated very near the 

 upper or anterior pole (fishes, 

 many insects), but it may be at the 

 opposite pole (some insects and 

 mollusks), or even on the side 

 (insects). In many insects there 

 is a group of half a dozen or more u- a h . f .1 c , 



o '^ Fig. 03. — Upper pole of the &ga of Argo- 



micropyles near the upper pole of uauta. [ussow.] 



the egg, and perhaps correlated The egg is surrounded by a very thick 



with this is the fact that several me'nbrane, perforated at m by the funnel- 



, , , shaped micropvle; below the latter lies the 

 spermatozoa enter the egg, though egg-nucleus in 'the peri-vitelline layer of pro- 

 only one is concerned with the toplasm ; /.i^. the polar bodies. 



actual process of fertilization. 



The plant-ovum, which is usually known as the oospJicrc (Figs. 64, 

 107), 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 struc- 

 ture 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 

 of the plant-ovum is the fact that it often contains plastids (leuco- 

 plasts or chromatophores) which, by their division, give rise to those 

 of the embryonic cells. These sometimes have the form of typical 

 chromatophores containing pyrenoids, as in Volvox and many other 

 Algae (Fig. 64). In the higher forms (archegoniate plants), according 

 to the researches of Schmitz and Schimper, the tgg contains numer- 

 ous minute colourless " leucoplasts," which afterward develop into 

 green chromatophores or into the starch-building amyloplasts. This 

 is a point of great theoretical interest ; for the researches of Schmitz, 

 Schimper, and others have rendered it highly probable that these 



