THE FROG AND VERTEBRATES IN GENERAL 205 



the center of the testis into the vasa efferentia. The other end of 

 each tubule is closed and lies near the outer wall of the testis. The 

 sperm develop from the primordial germ cells situated in the 

 walls of the tubules. The germ cells are typical in structure at 

 first, but after passing through the various stages (spermatagonia, 

 spermatocytes, and spermatids) they become greatly modified 

 cells each of which is made up of (a) a pointed head, containing 

 the male gametic nucleus ; (6) a middle piece, containing cyto- 

 plasm and centrosome, and (c) a long tail, or flagellum, which 

 has an active, vibratory movement, and enables the sperm cell to 

 move through a liquid with considerable rapidity. (W. f. 133, B.) 



Female Reproductive Organs. The female organs of repro- 

 duction consist of a pair of ovaries in which the eggs are devel- 

 oped, and a pair of oviducts which carry the eggs to the cloaca. 

 There is also a pair of fat bodies, one of which is attached to each 

 ovary. Each ovary is really a thin-walled sac with an outer 

 covering which is a continuation of the peritoneum lining the 

 entire body cavity. This outer covering encloses a layer of ger- 

 minal epithelium containing the primordial germ cells from 

 which the eggs arise. The eggs are more or less grouped and the 

 ovary as a whole has a number of well-defined lobes. The organ 

 is very plentifully supplied with blood vessels. (W. f. 132.) 



Each egg cell in the ovary is enclosed by a thin cell wall (vitel- 

 line membrane) which is surrounded by several layers of cells 

 (membrana granulosa). The ovarian egg with the surrounding 

 layers constitutes a Graafian follicle. The eggs arise from 

 cells which appear typical. They pass through various stages of 

 development and gradually increase in size until they are several 

 times the size of a somatic, or body cell. This growth results from 

 the gradual accumulation of reserve food material, and is a very 

 common phenomenon in the eggs of many animals. Except for 

 its large size, the mature egg shows a typical cell structure, and 

 is, therefore, very different from the highly modified sperm cell. 

 (W. f. 133, A.) 



The eggs of the Frog reach their full development at only one 

 period each year. This is usually in the spring, at which time the 

 two ovaries, distended with masses of eggs, fill up a large portion of 

 the abdominal cavity. When the proper time arrives, the eggs 

 break through the thin ovarian wall and are drawn into the open- 

 ings of the oviducts. There is no direct connection between the 



