946 SPECIAL PHYSIOLOGY. 



and more accessible to air and light. The yolk, a, inclosed within its 

 proper vitelline, or vitellary membrane, is of a pale-yellow color, and 

 even of a lighter specific gravity than water ; it is composed of 29 

 per cent, of fatty or oily matter, 17 of albumen, traces of phosphorized 

 fatty matter, of cerebrtc acid, and of salts, amounting in all to 2 per 

 cent., and of 52 per cent, of water. It presents a fluid basis, com- 

 posed of albumen in solution, mixed with fine granules ; in this are 

 contained larger granules, and also larger bodies, called yolk-vesicles. 

 These latter are not true cells, for they contain no nuclei ; they vary 

 from 4^o tn to 6jj tn f an i nc h ^ n diameter, and are composed chiefly 

 of fat particles aggregated together, but having no distinct cell-mem- 

 brane or envelope around them, though they may be covered by an 

 indistinct film of firmer albuminous substance. The outer parts of 

 the yolk, as may be seen when it is boiled, are laminated or stratified, 

 the several concentric layers being called halones or haloes, Fig. 119 ; 

 in its centre is a cavity, known as the central cavity or latebra, Fig. 

 119, in which the yolk is more fluid and contains true nucleated cells, 

 mixed with free oil-globules of different sizes, floating in an albuminous 

 fluid, and forming what is called the white yolk. This is an extension 

 of the germ-yolk. Leading from this central cavity to that surface of 

 the yolk which always floats uppermost, is a channel, also filled with 

 white yolk. At the upper end of this channel is the pale, circular 

 spot, or disc, known as the cicatricula or germinal disc, Figs. 116, 

 117. This, which is the essential part of the germ-yolk, consists, 

 even before the commencement of incubation, of two distinctly sepa- 

 rable layers: the upper layer consists of firm and clear substance; 

 the under layer, which is larger, is more opaque, and is composed of 

 nucleated cells. 



The Mammalian Ovaries and Ovum. 



In the Mammalia, the organs in which the ova are formed, the so- 

 called ovaries, are always double, one on each side. They are solid 

 and not racemose, as in the Bird, and are of small proportionate size, 

 corresponding with the smaller size of the holoblastic ova. They con- 

 sist of a firm, indistinctly fibrous, vascular stroma, containing numerous 

 vesicles, distended with a clear fluid, and named the Graqfian vesicles 

 QV follicles, homologous with the ovisacs of Birds. These vary in size, 

 from that of a pin's head to that of a pea, according to the stage of 

 their maturity. The walls of each Graafian vesicle, consist of an in- 

 closing vascular stroma, within which is a membrana propria, and, 

 within that an epithelial layer or layers, forming the membrana granu- 

 losa. Embedded in a part of this latter, named the proligerous disc, 

 is the minute holoblastic ovum, averaging about T J^th of an inch in 

 diameter. The size of the Human ovum varies from 2 io tn to iio tn 

 of an inch, that of the germinal vesicle, from 7 J^th to ^^th of an 

 inch in diameter. In the Mammalian ovary, the ovisac, or wall of the 

 Graafian vesicle, is not everywhere in close contact with the ovum, as 

 in Birds. 



At a certain period, a Graafian follicle bursts, not by a fissure, but 



