THE EGG-CELL OR OVUM. I03 



general facts can here be stated. The envelopes may be derived {a) from 

 the ovum itself, {/>) from surrounding cells, {c) from the secretion of special 

 glands. 



{a.) Just as a protozoon often exhibits distinct outer and inner zones, 

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

 ovum. What are called yolk or vitelline membranes are generally pro- 

 duced 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 

 inicropyle is sometimes present, through which the sperm enters, or nutri- 

 tive 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 {e.g.. Will), the follicular cells sometimes arise from within the 

 ovum, as the result of an early activity in the nucleus. This view, however, 

 cannot be said to be confirmed. 



{c.) As the ovum ripens, and passes from the ovary into the duct, it 

 often becomes surrounded by gelatinous, horny, limy, and other invest- 

 ments. In most cases, it necessarily follows that the egg has first been 

 fertilised. 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- 

 envelopes of reptiles, the firm limy egg-shells of birds, so often stained with 

 pigments, afford 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. 



§ 6. Birds' Eggs. — The student may be fitly directed to the 

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

 illustration of many facts. There he will see the great mass of 

 yolk, of two kinds, yellow and white, and on the top of this the 

 minute area of 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, Malpighi, 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 epigenesis ; and it is 

 this that the observers of to-day 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 of which the little air-chamber is formed ; and finally, the 

 hard but porous limy shell. There arises the difficult problem 

 of the origin of the shell, in regard to which it is to be noted that 



