AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



The Ovum. The human ovum was discovered in 1827 by Von Baer, and 

 it was he who first completely traced the connection between ova in the gene- 

 rative passages and ova in the Graafian 

 follicles of the ovary. The conception 

 of ova as the essential female element 

 had, however, long been held, and Har- 

 vey's dictum of the seventeenth century, 

 that everything living is derived from 

 an egg (omne vivum ex ovo), is well 

 known. The human ovum, as it comes 

 from the ovary, is a spherical, proto- 

 plasmic cell (Fig. 307), averaging with 

 the zona radiata, approximately 0.2 milli- 

 meter (y^ inch) in diameter. As in 

 other cells, the cell-body may be distin- 



FIG. 307.-Human ovum (modified from Na- <y U i sne d from the nucleus, the proto- 



gel): n, nucleus (germinal vesicle) containing t A 



the amoeboid nucleolus (germinal spot) ; d, deu- plasm of the former being Called Cyto- 



toplasmic zone; p protoplasmic zone; z, zona , j j fc fi gtructure the to _ 



radiata ; s, perivitelline space. * V 



plasm consists of an excessively delicate 



network of protoplasmic substance. As in other mammalian eggs, it proba- 

 bly contains, adjoining the nucleus, a minute, specially differentiated portion, 

 consisting of a single or double centrosome surrounded by an attraction sphere 

 (Fig. 308, A). For some distance inward from the border the cytoplasm is 

 pure and transparent, and this portion is often called the protoplasmic zone 

 (Fig. 307, 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 

 zone (d). 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 embryonic 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 nucleus (n), commonly 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. 



