I.] THE WHITE YOLK. 15 



destroyed by crushing. When boiled or otherwise hardened 

 in situ, they assume a polyhedral form, from mutual pressure. 

 The granules they contain seem to be of an albuminous 

 nature, as they are insoluble in ether or alcohol. 



Chemically speaking the yolk is characterized by the presence in large 

 quantities of a proteid matter, having many affinities with globulin, and called 

 vitellin. This exists in peculiar association with the remarkable body Lecithin. 

 (Compare Hoppe-Seyler, Hdb. Phys. Chem. Anal.) Other fatty bodies, 

 colouring matters, extractives (and, according to Dareste, starch in small quan- 

 tities), &c. are also present. Miescher (Hoppe-Seyler, Chem. Untersuch. p. 502) 

 states that a considerable quantity of nuclein may be obtained from the yolk, 

 probably from the spherules of the white yolk. 



FIG. 2. 



A. Yellow yolk-sphere filled with fine granules. The outline of the sphere 

 has been rendered too bold. 



B. White yolk-spheres and spherules of various sizes and presenting different 

 appearances. (It is very difficult in a woodcut to give a satisfactory repre- 

 sentation of these peculiar structures.) 



7. The yellow yolk thus forming the great mass of the 

 entire yolk is clothed externally by a thin layer of a different 

 material, known as the white yolk, which at the edge of 

 the blastoderm passes underneath the disc, and becoming 

 thicker at this spot forms, as it were, a bed on which the 

 blastoderm rests. Immediately under the middle of the 

 blastoderm this bed of white yolk is connected, by a narrow 

 neck, with a central mass of similar material, lying in the 

 middle of the yolk (Fig. 1, w. y.}. When boiled, or otherwise 

 hardened, the white yolk does not become so solid as the 

 yellow yolk; hence the appearances to be seen in sections 

 of the hardened yolk. The upper expanded extremity of 

 this neck of white yolk is generally known as the "nucleus of 

 Pander." 



Concentric to the outer enveloping layer of white yolk 

 there are within the yolk other inner layers of the same 

 substance, which cause sections of the hardened yolk to 



