THE OVUM. 



fTlHE whole body is developed out of the ovum or female element (Figs. 687 and 

 .L 688) after it has been fertilized by the spermatozoon or male element. The 

 ovum is a simple nucleated cell, and all the complicated changes by which the 

 various intricate organs of the body are formed from it may be reduced to two 

 general processes, viz., the segmentation or cleavage of cells, and their differentia- 

 tion. The former process consists in the division of the nucleus and the surround- 

 ing cell-substance, whereby the original cell is represented by two. The differentia- 

 tion of cells is a term used to describe that unknown power or tendency impressed 

 on cells, apparently identical in structure, whereby they grow into different forms ; 

 so that (to take one of the first phenomena which occurs in the 

 growth of the embryo) the indifferent cells of the vascular area 

 are differentiated, some of them into blood-globules, others into 

 the solid tissue which forms the blood-vessels. The extreme 

 complexity of the process of development renders it at all times 

 difficult to describe intelligibly, and still more so in a work like 

 this, where adequate space and illustration can hardly be afforded, 

 having respect to the main purpose of the work, and therefore 

 an outline of the principal facts onlv will be given. Many of 



, " ". > FIG. 687. Human 



the statements which are accepted in human embryology are ovum from a middie- 



j ,i ,1 f i , L -u i. : x it, sized follicle. Mugni- 



made on the strength of what has been observed to occur in the tied 350 times, a.zcma 

 lower animals, and their existence in the human subject is radiata d& bExtenwa 

 merely a matter of inference. Within recent years, however, border of the yolk and 



J , ill -IT fii-i f internal border of the 



much has been added to our knowledge of the development of viteiiine membrane. 



,i_ i i T .1 11 i ,i_ c. Germinal vesicle 



the human embryo, and this more especially by the important au d germinal spot, 

 researches of Professor His and others. 



The ovum is a small spheroidal body situated in the immature Graafian follicle 

 near its centre, but in the mature one in contact with the membrana granulosa, 1 

 at that part of the follicle which projects from the surface of the ovary. The cells 

 of the membrana granulosa are accumulated round the ovum in greater number 

 than at any other part of the follicle, forming a kind of granular zone, the discus 

 proligerus. 



The human ovum (Fig. 687) is extremely minute, measuring from y^ to T | T of 

 an inch in diameter. It is a cell consisting externally of a transparent striated 

 envelope, the zona pellucida, zona radiata, or viteiiine membrane. The extra- 

 nuclear protoplasm contained within the zona pellucida is known as the cyto- 

 plasm ; it is a sponge-like material, containing in its meshes numerous large 

 fatty and albuminous granules, which constitute the yolk or vitellus ; in the 

 neighborhood of the nucleus, however, these granules are comparatively few in 

 number. The nucleus is a large spherical body, which is known by the name 

 of the germinal vesicle, and resembles in structure the nucleus of an ordinary 

 cell. Within it there is generally one nucleolus, which is large and well marked, 

 and is known as the germinal spot. The zona pellucida is believed to be pierced 

 by numerous pores which are probably channels of nutrition and which give it 



1 See the description of the ovary at a future page. 



1149 



