THE EARLY DEVELOPMENT OF THE FROG 77 



Entrance of the eggs into the upper part of the oviduct 

 stimulates the jelly-secreting glands of its walls, and as the eggs 

 are carried along singly down the oviduct by the cilia of its 

 own walls, each is smeared over the surface of the chorion with 

 a thin coating of viscid albuminous material arranged in two 

 or three layers. About two hours are occupied in the passage 

 of an ovum down the duct. At the lower ends of the oviducts 

 the eggs collect in the uteri, where they remain stored, usually 

 for a day or two, pending the time of spawning. 



3. Spermatogenesis 



The formation and maturation of the spermatozoa is com- 

 pleted within the testes. Each lobule of the testis is composed 

 of a collection of tubules, in the walls of which the spermato- 

 gonia develop, surrounded by nutritive follicles the elements 

 of which become in part the basal cells or Sertoli cells. So 

 far as is known the formation of the spermatocytes and sper- 

 matids is fairly typical. The spermatid contains a large 

 nucleus and two peripheral centrioles. During the metamor- 

 phosis of the spermatid into the spermatozoon, the inner 

 centriole is taken into the nucleus while opposite the other the 

 flagellum grows out. The sphere of idioplasm remains on one 

 side of the anterior tip of the head, when this forms from the 

 nucleus, and a part of the cytoplasm flows down around the 

 base of the flagellum forming the middle piece; the remainder 

 of the cytoplasm appears to be thrown off. 



In some species of Rana the sperm form continuously, in 

 others only seasonally, apparently just before hibernation 

 begins. As the breeding season approaches they are produced 

 more abundantly and collect in the dilated lower ends of the 

 vasa deferentia or seminal vesicles, ready for extrusion. 



C. SPAWNING 



In the more common species of Rana, spawning occurs 

 during the first warm days of early spring; some forms spawn 



