OOGENESIS 



2 9 



in their nuclear constitution, they differ radically as to form, size, and cyto- 

 plasmic structure. The ova are very large cells, stored with nutriment 

 for the embryo which each one may later produce. In the higher verte- 

 brates they are formed in relatively small numbers. According to Hen- 

 sen's estimate, about two hundred, ready for fertilization, are produced 

 by the human female in a life-time. But the male, according to Lode, 

 produces 340 billion spermatozoa, or, as stated by Waldeyer, nearly 850 

 million per ovum. A large number must be produced, since many will 

 fail to traverse the uterus and tube so as to find the ovum at the time of 

 its discharge from the ovary. The ova of lower vertebrates, which 

 are fertilized and develop outside of the body, are discharged in great 

 numbers; in certain fishes from three to four million are produced 

 annually. 



The multiplication of oogonia in the human ovary takes place before 

 birth, and about fifty thousand are produced. At birth, or shortly there- 

 after, all the oogonia have become primary ob'cytes (Keibel). At first the 

 oocytes are small, but they enlarge at varying rates, and the largest are 

 indistinguishable from mature ova except by their nuclear contents. Since 

 some grow more rapidly than others, the ovary in childhood contains 

 primary oocytes of many sizes. Each oocyte becomes enclosed in a cyst 

 or follicle. The way in which these follicles develop, and the. manner in 

 which the oocyte escapes into the uterine tube by the rupture of these 

 follicles, will be described in connection with the ovary. Between the cells 

 of the follicle and the oocyte, there is a broad, radially striated membrane, 

 known as the zona pellucida or zona radiata (Fig. 2 2) . This zona has some- 

 times been regarded as a cell membrane, but the oocyte divides within it as 

 if enclosed in a capsule. It does not invest the daughter cells like a mem- 

 brane. The radial striations have been interpreted as slender canals 

 containing processes of the f ollicular cells, and the zona has been considered 

 as a product of these cells. In certain cases a perivitelline space has been 

 described as encircling the oocyte and thus separating it from the zona, 

 but this space has been considered as artificial, or as a refractive line 

 wrongly interpreted as a space. 



The cytoplasm of the oocyte becomes charged with yolk granules or 

 spherules. They constitute the deuteroplasm (or deutoplasm), but this 

 term is equally applicable to fat droplets and other secondary products of 

 the protoplasm. In the human oocyte the granules are centrally placed 

 (Fig. 22), and they are so transparent, when fresh, as to cause only a slight 

 opacity. In the eggs of many animals the yolk is more highly developed, 

 and it may be evenly distributed or gathered at one pole. Within the 

 cytoplasm of the developing oocyte, a large dark body of radiate structure 

 is sometimes conspicuous. It is inappropriately known as the yolk nucleus, 

 and is probably a derivative of the centrosome and surrounding archo- 



