454 THE GLANDULAR ORGANS. 



directly in the egg itself. These products have been studied in 

 some detail in the foregoing pages, and we have seen that they are 

 in part, at least, specific of the egg, and do not occur elsewhere in 

 the animal body. This holds good more especially of the albu- 

 mins, and it follows that all those forms that enter into the com- 

 position of the various tissues must of necessity be produced from 

 the pre-existing forms during the development of the young animal. 

 The fats may, in part, be utilized directly in the construction of the 

 fats of the embryo, but to a large extent, no doubt, they represent 

 the principal form of energy which is placed at the disposal of the 

 developing organism. Carbohydrates, as such, are practically lack- 

 ing among the food-stuffs of the egg, and must hence be formed 

 synthetically. That glycogen can be demonstrated in the tissues 

 of the embryo at a very early date, has already been stated, which 

 proves in itself that the animal organism is not dependent upon 

 the ingested carbohydrates for its glycogen supply. As nucleopro- 

 teids, moreover, do not occur in the egg, it would appear that these 

 also must be formed from other albuminous substances, and there 

 can be little doubt that the nucleo-albumins are here of prime impor- 

 tance. In this connection it is interesting to note, however, that 

 Mesernitzki found xanthin-bases in identical amounts in non-incu- 

 bated eggs, as in early chick-embryos. The salts which are required 

 by the developing organism are, as has been seen, present in the 

 egg in abundance. 



The essential factor, however, which is necessary to development 

 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- 



