Growth and Development of the Body. 



Yelk 



Camaud \ 

 Gtnnnd spot' 



-JJi'scusfrdy 



Ovum of the sow. 



THE whole body grows out of the fecundated ovum, and it is accordingly 

 necessary to follow, in as few words as possible, the various stages in which 

 the ovum is found in the uterus, from the earliest moment at which conception 

 can be recognized as having taken place, down to the birth of the complete 

 foetus. The ovum is a small spherical body, situated in immature Graafian 

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

 granulosa (see Ovum in body of the work), 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 proliger us (Fig. 49). 



The human ovum (Fig. 50) is extremely minute, 

 measuring from u^th to -ri^th. of an inch in 

 diameter. It is a cell, consisting externally of a 

 transparent envelope, the zona pellucida or vitel- 

 line membrane. Within this, and in close contact 

 with it, is the yelk or vitellus ; imbedded in the 

 substance of the yelk is a small vesicular body, 

 the germinal vesicle (vesicle of Purkinje) the 

 nucleus of the cell ; and this contains as its nucle- 

 olus a small spot the macula germinativa, or the 

 spot of Wagner. 



The zona pellucida, or vitelline membrane, is a thick, colorless, transparent 

 membrane, which appears under the microscope as a bright ring, bounded 

 externally and internally by a dark outline. It corresponds to the chorion of 

 the impregnated ovum. 



The yelk consists of granules and globules 

 of various sizes, imbedded in a more or less 

 viscid fluid. The smaller granules resemble 

 pigment ; the larger granules, which are in 

 greatest number at the periphery of the yelk, 

 resemble fat-globules. In the human ovum, 

 the number of granules is comparatively 

 small. 



The germinal vesicle consists of a fine, trans- 

 parent, structureless membrane, containing a 

 watery fluid, in which are occasionally found 

 a few granules. It is about 7 i n of an inch in 

 diameter, and in immature ova lies nearly in 

 the centre of the yelk ; but, as the ovum be- 

 comes developed, it approaches the surface, and enlarges much less rapidly 

 than the yelk. 



The germinal spot occupies that part of the periphery of the germinal 

 vesicle which is nearest to the periphery of the yelk. It is opaque, of a 

 yellow color, and finely-granular in structure, measuring from s ^ to 2 ^ 50 of 

 an inch. 



The phenomena attending the discharge of the ova from the Graafian 

 vesicles, since they belong as much or more to the ordinary functions of the 

 ovary than to the general subject of the growth of the body, are described 

 with the anatomy of the ovaries in the body of the work. 



Fig. 50. 



Human ovum, from a middle-sized foi- 

 licle (magnified 250 times), a. Vitelline 

 membrane. Zona- pellucida. b. External 

 border of the yelk and internal border of 

 the vitelline membrane, c. Germinal vesi- 

 cle and germinal spot. 



