THE EGG-CELL OR OVUM. 97 



In other cases, especially in insects, the ovum grows rich at the expense of 

 neighboring cells, which are sacrificed to its nutritive equipment. But it is evi- 

 dent enough that a cell remains a cell, however many of its neighbors it may 

 happen to absorb. 



V. Egg-envelops. — The ovum starts as a naked cell, but generally becomes 

 furnished with ensheathing envelops. The exact history of the egg-membranes 

 and sheaths is a very complex matter. Only the most general facts can here be 

 stated. The envelops may be derived (a) from the ovum itself, {b) from 

 surrounding cells, (c) from the secretion of special glands. 



(a) Just as a protozoan often exhibits distinct outer and inner zones, 

 distinguished by minor physical and chemical peculiarities, so it is with the 

 ovum. What are called yelk or vitelline membranes are generally produced by 

 the ovum itself. Furthermore, the outer protoplasm often forms a distinct firm 

 zone, known as the zona pellucida. This may be traversed by fine radiating 

 pores establishing nutritive communication with the exterior, and is then known 

 as the zona radiata. A special aperture or micropyle is sometimes present, 

 through which the sperm enters, or nutritive supply is sustained. 



(b) The ovum, in its young stages, is very frequently seen surrounded by a 

 circle of small cells, which form what is called a follicle. These may produce a 

 membrane or a glairy investment. According to some investigators (for 

 example, Will), the follicular cells sometimes arise from within the ovum, as the 

 result of an early activity in the nucleus. This view, however, can not be said 

 to be confirmed. 



(<r) As the ovum ripens, and passes from the ovary into the duct, it often 

 becomes surrounded by gelatinous, horny, limy, and other investments. In 

 most cases, it necessarily follows that the egg has first been fertilized. The 

 investments are usually referable to the activity of the walls of the oviduct or 

 uterus, though sometimes there are special shell-glands, and the like. The 

 chitinous cases of some insect-ova, the horny mermaids' purses of many gristly 

 fishes, the more or less limy egg-envelops of reptiles, the firm limy eggshells of 

 birds, so often stained with pigments, aftord good illustrations of these secondary 

 investments. Quite distinct are cocoons, such as those of earthworm and leech, 

 which surround several eggs, and are produced from the skin of the animal. 



VI. Birds' Eggs. — The student may be fitly directed to the egg 

 of the fowl, or of some other bird, for a convenient concrete illustra- 

 tion of many facts. There he will see the great mass of yelk, of two 

 kinds, yellow and white, and on the top of this the minute area ot 

 formative protoplasm. It was on this, as it gradually revealed the 

 cloudy outlines of the embryo chick, that the Greeks looked with 

 naive unaided eyes. Here it was that Aldrovandus, Harvey, Mal- 

 pighi, Haller, and the early embryologists, with clear vision, saw 

 almost as much as their appliances would permit. It was this which, 

 in its primitive simplicity, impressed Wolff with the reality of epigen- 

 esis ; and it is this that the observers of today look down upon 

 through their embryoscopes, or cut sections of with their microtomes. 

 Then round about all is the secondary investment of "'white of egg" 

 or albumen ; round this a shell-membrane, between the two layers 01 

 which the little air-chamber is formed ; and finally, the hard but 



