150 AiX AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



(Fig. 220, p). Throughout the centre of the cell, however, it is obscured by 

 the presence of an abundance of yolk-substance, or deutoplasm, from which 

 the corresponding part of the ovum is sometimes called the deutoplasmic 

 /one ((/). Deutoplasm is non-living substance; it consists of granules of 

 yolk imbedded in the meshes of the cytoplasmic network, and, like its ana- 

 logue, the yolk of the hen's egg, it serves as food for the future cells of the 

 embryo. 



A comparison of the respective amounts of food in the human and the 

 fowl's egg, with the manner of embrvonic development, is suggestive. The 

 chick develops outside the body of the hen, and, therefore, requires a large 

 supply of nutriment, which it finds in the yolk and the white of the egg. The 

 child develops within the mother's body and receives its nourishment from the 

 maternal blood; hence the supply of food within the egg is only enough to 

 ensure the beginning of growth, special blood-vessels being formed to facilitate 

 its continuance. 



The nudevs (/<), frequently called by its early name, the germinal vesicle, is 

 spherical, and usually occupies a slightly eccentric position. Its protoplasm 

 consists of a network composed of two kinds of material : the more delicate, 

 slightly staining threads are the achromatic substance, the coarser, deeply 

 staining portion, the chromatic substance or chromatin. The former is con- 

 tinuous with, and probably of exactly the same nature as, the cytoplasm. 

 The chromatin is peculiar to the nucleus, and at certain stages in the nuclear 

 history is resolved into distinct granules or filaments, the chromosomes (Fig. 

 221, A), the number of which in the human ovum before maturation is 

 thought to be sixteen. There is every reason lor believing that the chromatin 

 is the bearer of whatever is inherited from the mother. The nucleus is 

 limited by a nuclear membrane, and contains a strongly marked nucleolus, 

 which has likewise retained its original name of germinal spot. There is 

 probably no proper cell-wall, or vitelline membrane, such as is said to exist in 

 many mammalian and other eggs. The ovum is, however, surrounded by a 

 thick, tough, transparent membrane of ovarian origin, about 0.02 millimeter 

 (rsViJ mcn ) m thickness, and called the zona radiata or zona pellucida (Fig. 

 220, z). It is pierced by a multitude of fine lines radiating from the surface 

 of the zona to the ovum ; these are thought to represent pores, to contain fine 

 protoplasmic processes of the surrounding ovarian cells, and thus to serve as 

 channels Cor the passage of nutriment to the egg. Between the zona radiata 

 and the ovum a narrow space, the perivitelline space (x), exists. Attached to 

 the outside of the zona r<i<li<it<i are usually patches of cells derived from the 

 discus proligerus of the Graafian follicle of the ovary, which may form a com- 

 plete covering and constitute the c<>i-<>,i<i radiata. They disappear soon after 

 the egg is discharged from the ovary. 



Regarding the chemistry of the mammalian ovum little is known definitely, 

 and of the human ovum nothing whatever except by inference from the eggs 

 of lower animals. The protoplasmic basis undoubtedly resembles other undif- 

 ferentiated protoplasm in its general composition, with an abundance of proteid 



