430 THE GLANDULAR ORGASS. 



on the filter is dissolved in as little ether as possible, and precipi- 

 tated with acetone. The precipitate is collected on a filter and 

 washed with acetone until the wash-acetone dissolves no more 

 cholesterin. The residue is again dissolved in a small amount of 

 ether or benzol. To this solution an excess of absolute alcohol is 

 added, when on standing a white amorphous substance separates 

 out, which can be obtained in crystalline form by solution in hot 

 alcohol and cooling; this apparently consists of tripalmitin. Alter 

 iiltration the pure lecithin can then be obtained from the ether- 

 alcoholic solution by precipitating with acetone, as before, or by dis- 

 tilling off the alcohol and ether. The resulting material is dried in 

 the vacuum. Its phosphorus varies between 3.7 and 4.1 per cent, 

 in amount. 



Incubation. Of the chemical changes which take place during 

 the process of fertilization, and in which the nucleus of the ovum is 

 primarily concerned, we know nothing. But there can be no doubt 

 that, in fishes at least, the protamin radicle of the nucleins of the 

 spermatozoa plays an important part. As a result, the reproductive 

 function of the ovum, which previously has remained dormant, 

 now manifests itself in the mysterious morphological changes which 

 the cell undergoes, and which end in the production of an organism 

 that is morphologically and chemically like its parents. 



In mammals the food-stuffs which are required by the devel- 

 oping organism are constantly supplied through the blood of the 

 mother-animal, but in the lower forms of life they are furnished 

 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, Avhich 

 proves in itself that the animal organism is not dependent upon 

 the ingested carbohydrates for its glycogen supply. As nuclear 

 nucleins, moreover, do not occur in the e<rg, it follows that these 

 also must be formed from other albuminous substances, and there 

 <"in be little doubt that the paranucleins are here of prime impor- 

 tance. 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 



