ORIGIN AND DEVELOPMENT OF THE BODY. 



THE whole body is developed out of the ovum (Fig. 72) when fertilized by the 

 spermatozoon, the ovum being merely a simple nucleated cell. All the 

 complicated changes by which the various intricate organs of the whole body are 

 formed from one simple cell may be reduced to two general processes viz. the 

 segmentation or cleavage of cells, and their differentiation. The former process 

 consists in the splitting of the nucleus and its surrounding protoplasm, whereby 

 the original cell is represented by two. The differentiation of cells is a 

 term used to describe that unknown power or tendency impressed on cells 

 which, to all methods of examination now known, seem absolutely identical, 

 whereby they grow into different forms ; so that (to take the first instance 

 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 develop- 



Zona peUucida. 

 Yolk.., 

 Germinal vesicle. 



Germinal spot. 



Discus prolig. 



FIG. 72. Ovum of the sow. 



FIG. 73. Human ovum from a mid- 

 dle-sized follicle. Magnified 350 times. 



a. Vitelline membrane or zona peUucida. 



b. External border of the yolk and 

 internal border of the vitelline mem- 

 brane, c. Germinal vesicle and germi- 

 nal spot. 



ment renders it at all times difficult to describe 

 it 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. I can only hope to render the leading features of the pro- 

 cess tolerably plain, and must refer the reader who wishes to follow the various 

 changes more minutely to the special works on the subject, and especially the 

 works of Minot and Hertwig. Many of the statements which are accepted in 

 human embryology are made only on the strength of observations on the lower 

 animals, many stages in the development of the human embryo being yet unknoAvn 

 to us. 



The ovum is a small spheroidal body situated in immature Graafian vesicles 

 near their centre, but in the mature ones in contact with the membrana granulosa ' 

 at that part of the vesicle 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 vesicle, forming a kind of granular zone, the discus 

 prolic/erus. 



The human ovum (Fig. 73) is extremely minute, measuring from -%^ to y|-g- of 

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

 the zona peUucida or vitelline membrane. Within this, and in close contact w r ith 

 it, is the cell-protoplasm containing granules of yolk or vitellus ; imbedded in the 



100 



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



