20 THE DEVELOPMENT OF THE CHICK 



about 0.07 mm. The larger spheres of the white yolk contain 

 several highly refractive granules of relatively considerable size 

 as compared with those of the yellow spheres (Fig. 3), and such 

 granules may have secondary inclusions. As we shall see later, 

 the smaller granules of the white yolk extend into the germinal 

 disc (forerunner of the blastoderm) and grade into minute yolk- 

 granules contained within the living protoplasm. 



The earlier investigators from the time of Schwann regarded the 

 white yolk-spheres as actual cells (Schwann, Reichert, Coste, His). 

 His especially laid great stress on this interpretation; he believed that 

 they were derived from the cells of the ovarian follicle which migrated 

 into the ovum in the course of ovogenesis, that they multiplied like other 

 cells, and took part in the formation of certain embryonic tissues. Sub- 

 sequently he abandoned this position as untenable. The white yolk 

 spheres are now universally regarded as food matters of a particular sort. 



The yolk and albumen are complex mixtures of many difTerent 

 substances, organic and inorganic, containing all the elements 

 necessary for the growth of the embryo. Very little is known 

 concerning the series of chemical changes that go on in them 

 during incubation. 



Chemical Composition of the Hen's Egg. — The following data 

 on the chemical composition of the hen's egg are taken from 

 Simon's Physiological Chemistry. For details and literature the 

 student is referred to the standard text-books of jDhysiological 

 chemistry. 



GENERAL COMPOSITION OF THE YOLK 



PER CENT. 



Water 47.19-51.49 



Solids 48.51-42.81 



Fats (olein, pahnitin, and stearin) 21.30-22.84 



Vitelline and other albumens 15.63-15.76 



Lecithin 8.43-10.72 



Cholesterin 0.44- 1.75 



Cerebrin 0.30 



Mineral salts 3.33- 0.36 



Colorine: matters 1 r> --o 



^1 \ O.oo3 



Glucose J 



Analysis of the Mineral Salts 



Sodium (NasO) 5.12- 6.57 



Potassium (K/J) 8.05- 8.93 



Calcium (CaO) 12.21-13.28 



