THE REPRODUCTIVE GLANDS. 431 



after fertilization, is an abundant supply of oxygen, and a tempera- 

 ture of about 40 C. The requisite amount of oxygen is obtained 

 from the air by a process of diffusion through the shell. In return 

 carbon dioxide is eliminated, together with a small amount of 

 nitrogen. These respiratory changes are but slight in the begin- 

 ning, but gradually increase. Water also is given off, and, as a 

 result, the weight of the egg diminishes. The increase of the solids 

 in the developing animal is, of course, accompanied by a correspond- 

 ing diminution of those of the egg itself. 



Systematic chemical examinations of the ovum in its various 

 stages of development have thus far not been made. Liebermann 

 appears to be the only one, indeed, who has attempted the problem. 

 His principal results may be summarized as follows : during the 

 first stage of development tissues are formed, which are very rich in 

 water ; later, however, the amount of water decreases. The abso- 

 lute amount of substances which are soluble in water steadily in- 

 creases, while their relative amount diminishes as compared with 

 the remaining solids. After the fourteenth day a large increase in 

 the amount of fat is noted, while previously this remains fairly con- 

 stant. The amount of soluble albumins and albuminoids increases 

 steadily and in such a manner that the absolute quantity increases, 

 while their relative amount remains nearly constant. Up to the 

 tenth day no collagen is found, but after the . fourteenth day a sub- 

 stance is present which on boiling with water yields a material sim- 

 ilar to cartilaginous glut in. A mucinous substance is found about 

 the sixth day, but it subsequently disappears. The amount of 

 haemoglobin steadily increases in its relation to the body-weight. 



The chemical composition of the allantoic fluid and the amniotic 

 fluid has already been considered (page 344). 



The placenta has not as yet been studied in detail, but it is likely 

 that its greater portion consists of collagen, in accordance with its 

 fibrous structure. In its marginal zone two pigments have been 

 encountered which apparently are related closely to bilirubin and 

 biliverdin, and are derivatives of haemoglobin. The orange pigment 

 may be obtained in crystalline form, while the green pigment, which 

 has been termed hcwnatochlorine, is amorphous. 



