EARLY DEVELOPMENT. 



The human body with all its complex organism is the product of the differentia- 

 tion and specialization of the cells resulting from the union of the parental sexual 

 elements, — the ovum and the spermatozoon. 



The Ovum. — The maternal germ-cell is formed within the female sexual gland, 

 the ovary, in which organ it passes through all stages of its development, from the 

 immature differentiation of its early condition to the partially completed matura- 

 tion of the egg as it is liberated from the ovary. 



The human oznini, in common with the ova of other mammals, is of minute 

 size, being, as it is discharged from the ovary, about . 25 millimetre in diameter. Ex- 

 amined microscopically and after sectioning, the human ovum is seen to be enclosed 

 within a distinct envelope, the zo7ia pellucida, .014 millimetre in thickness, which 

 in favorable preparations exhibits a radial striation, and hence is also named the 

 zo7ia radiata. This envelope at first was confounded with the proper limiting mem- 

 brane of the cell, and for a time was erroneously regarded as corresponding to the 



Fig. II. 



-Corona radiata 



Zona pellucida 



Germinal vesicle (nu- 

 cleus ) containing germ- 

 inal spot (nucleolus) 



Zone rich in deutoplasm 



-Zone poor in deutoplasm 

 fading into homogene- 

 ous peripheral zone 



Human ovum from ripe Graafian follicle. X 170 (Nagel.) 



cell-wall. The nature of the zona pellucida is now generally conceded to be that of 

 a protecting membrane, produced through the agency of cells surrounding the 

 ovum. 



The substance of the ovum, the yolk, or vitellus, consists of soft, semifluid pro- 

 toplasm modified by the presence of innumerable yolk-granules, the representatives 

 of the important stores of nutritive materials present in the bird's egg. Critically 

 examined, the vitellus is resolvable into a reticulum of active protoplasm, or ooplasm, 

 and the nutritive substance, or deutoplasm. At times the yolk is limited externally 

 by a very delicate envelope, the vitellhie membrane, which usually lies closely placed, 

 or adherent, to the protecting zona radiata ; sometimes, however, it is separated 

 from the latter by a perivitelline space. The vitelline membrane is probably absent 

 in the unfertilized human ovum. 



A large spherical nucleus, the germinal vesicle, approximately .037 millimetre in 

 diameter, usually lies eccentrically within the yolk, surrounded by the distinct nuclear 

 membrane. Within the germinal vesicle the constituents common to nuclei in 



15 



