378 EMBRYOLOGY. 



egg-germs are found, even in the adult animal, sometimes in the 

 most widely separated parts of the ovary, sometimes limited to 

 definite regions of the gland. In the second case the period of 

 forming primitive ova in the germinal epithelium bears a direct 

 ratio to the total number of ova eliminated during the life of the' 

 individual. Thus Waldeyer states concerning Man that in the 

 second year after birth the formation of new ova can no longer be 

 shown. 



Nevertheless in Man the number of ova contained in a single 

 ovary is very great. They have been estimated to number in a 

 sexually mature girl 36,000. In other Mammals the production of 

 new ova appears to last longer. Pfluger's tubes which were still 

 connected with the germinal epithelium and contained small pri- 

 mordial ova have been observed even in young animals (Dog, Eabbit, 

 etc.). However, it has been questioned whether we here have 

 really new structures or only primitive ova that in their development 

 have remained stationary. It is maintained by van Beneden with 

 certainty for a few Mammals, e.g., the Bat, that in the sexually 

 mature animal Pfluger's tubes and primitive ova still continue to- 

 be produced from the germinal epithelium. 



In connection with the first formation of the follicle I will here 

 add some statements about its further metamorphosis. This is very 

 similar in the different Vertebrates, excepting Mammals. 



In most Vertebrates the follicle (fig. 216 /) consists at first of a, 

 small, centrally located egg-cell and a single layer of small follicular 

 cells enveloping it. Soon both are more sharply separated from 

 each other by means of a vitelline membrane. In older follicles 

 both parts have increased in size. The follicular cells ordinarily 

 grow out into long cylinders, and appear to play an important part 

 in the nutrition of the egg. In many animals, e.g., in Sharks and 

 Dipnoi, yolk-granules have been found in them, as in the egg itself, 

 and it has been concluded from this, as well as from other phenomena, 

 that the follicular cells take up nutritive substance from the vas- 

 cular follicular capsule, and pass it along to the egg. Such a method 

 of nutrition is made easier by the fact that the vitelline membrane 

 (fig. 5 z.p) is traversed by tubules, through which the follicular 

 cells {f.z) send protoplasmic filaments to the egg. When the egg 

 has attained its full size, the follicular cells lose their significance as 

 nutritive organs and become more and more flattened. 



In the lower Vertebrates the mature ova are generally eliminated 

 in great numbers all at once, frequently in the course of a few days- 



