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 (lig. 3), and such 
granules may have secondary inclusions. As we shall see later, 
the smaller granules of the white yolk extend into the germinal 
dise (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 different 
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 physiological 
chemistry. 
GENERAL COMPOSITION OF THE YOLK 
PER CENT. 
A225) ae ee ee ee eee ee 47.19-51.49 
elolbte kin Reis Ga ean a aan anes cae 17 OYE oe Se 48.51-42.81 
Fats (olein, palmitin, and stearin) ................. . 21.30-22.84 
Vitelline and other albumens ....... eer 15.63-15.76 
eGrGhint wegen tee es 8.43-10.72 
Cholesterin ....... meee Sonat Ae le 0.44— 1.75 
Cerebrin ........... ha a S Gt are 0.30 
Mineral salts ....... , on fh 3.33- 0.36 
Coloring matters | ce 
be Weenehe epee hee Ment oye ORE) Sees semen aiteee ra tee Lente 0.553 
Glucose 
ANALYSIS OF THE MINERAL SALTS 
Jee see ee 5.12— 6.57 
«ante ee ca aT 2a, ae ee eine 8.05— 8.93 
ee tge 6 ects Be eee ae setcwenss “LAZINESS 
Sodium (NasO) ... 
Potassium (KK,Q) . . 
Caleium (CaO) 
