196 



THE FROG 



embryo. It is covered with a delicate membrane, the vitel- 

 line inemhrane. By the time the egg is mature a superficial 

 deposit of pigment takes place over one hemisphere. 



In the young condition all the epithelial cells forming the 

 walls of the ovary are alike, but as the organ reaches 

 maturity, certain of them {0) enlarge, and give rise to the 



Fig. 63 — Transverse section of frog's ovary. 

 /'. 71. blood vessels ; c. t. connective tissue ; c/. outer layer, and ej>' , Inner layer of 

 epithelium ; ep" . outer layer of ovisac, continuous with cp' .\ cf>"'. germinal epi- 

 thelium, derived from cfi ; r/"". foUicul.ar epithelium, derived from ep ; ?iii. 

 nucleus of ovum ; 7iu'. nucleoli ; o. young ovum ; pr. protopLasm of ovum 

 containing yolk-granules. (After Marshall.) 



ova, while others form an investment or Collide (ef") 

 for each ovum. 



The oviduct (Fig. 4, r. ovd), as you have seen, is a 

 long and greatly convoluted tube lying above or dorsal to 

 the ovary. Its anterior end narrows considerably, runs 

 parallel with the gullet, passes to the outside of the root of 

 the lung, and then opens into the ccelome by a small aper- 

 ture (/-. ovd''-). The greater part of the oviduct is about as 

 wide as the small intestine, and is thick-walled and lined 



