THE EARLY DEVELOPMENT OF THE FROG 73 



folds of peritoneum attached just along the outer sides of the 

 mesovaria (Fig. 24). Each oviduct opens directly out of the 

 body cavity at its upper end by a ciliated ostium. and at its 

 lower extremity it opens into the cloaca. The thickness of the 

 oviducal wall is due chiefly to the presence of glands secreting 

 the albuminous material which forms the outer egg membrane 

 or jelly; the lumen of the duct is lined with ciliated epithelium. 

 Both the length of the oviducts and the thickness of their 

 walls are subject to seasonal variation, the glands being 

 largest and the ducts most convoluted during the time of egg- 

 laying. The lower extremities of the oviducts are thin walled 

 and easily dilatable forming the so-called "uteri," serving as 

 storage spaces for eggs ready to be laid. 



In the male there is a single pair of ovoid testes (Fig. 25), in 

 a position corresponding to that of the ovaries, and similarly 

 suspended from the dorsal body wall by a peritoneal fold 

 (mesorchium). Each testis is drained by a variable number 

 (ten to twelve usually) of vasa efferentia which, after penetrat- 

 ing the kidney, open into a longitudinal collecting duct the 

 vas deferens (Wolffian duct) which serves also as a ureter. Just 

 before the vasa deferentia open into the cloaca they are dilated 

 into the seminal vesicles, where mature sperm are stored just 

 previous to spawning. The testes are divided into lobes like 

 the ovaries and each lobe is further subdivided into lobules, in 

 the walls of which the sperm develop and mature. There are 

 fat bodies in the male similar to those in the female (Fig. 25). 

 The testes also show much the same seasonal variation in size 

 as the ovaries, in some species in which the formation of sper- 

 matozoa is seasonal, while in others, which form the sperm 

 continuously, there is less variation. 



2. Oogenesis 



During the later ovarian history of the eggs the maturation 

 processes are commenced and the deutoplasm or yolk material 

 is accumulated (growth period). The nucleus of the early 

 primary oocyte passes into synizesis on that side of the nucleus 



