138 ANIMAL BIOLOGY. [Part I. 



sent a remnant of the mesonephros, and the Wolffian duct 

 becomes the vas deferens of the male, while it atrophies more 

 or less completely in the female. The rest of the Wolffian 

 body is generally held to atrophy more or less completely in 

 both sexes. 



Thus in the male the Miillerian ducts atrophy, but the 

 Wolffian ducts become the vasa deferentia, just as they do in 

 the frog, only in that type they have also the functions of 

 ureters. In the female it is the Wolffian ducts which atrophy, 

 while the Miillerian ducts become the oviducts. In the fowl 

 one usually the right atrophies. In the rabbit the oviducts 

 of the two sides unite in the median line to form the vagina. 



The embryo rabbit has, like the adult frog and fowl, a cloaca 

 which receives the excreta from the alimentary canal, and the 

 products of the kidneys and genital organs. This, however, 

 becomes modified in such a way as to separate the opening of 

 the rectum from that part (the urino -genital sinus) which receives 

 the ureters and the genital ducts. Further changes occur which 

 differ in the two sexes : one of the most remarkable of which is 

 the passage of the testes from the peritoneal cavity into two 

 special pouches of skin, the scrotal sacs. 



The Heart. This organ is formed very early by the coales- 

 cence of the layers of splanchnic mesoblast beneath the throat. 

 The cleavage of the mesoblast on each side of the ^body into a 

 splanchnic and a somatic layer is shown in Fig. 38A, i. This 

 section is through the mid region of the body. Further forward 

 the mesenteron is closed in below and converted into a tube, 

 beneath which the splanchnic layers come together. But they 

 do so in such a way as to leave a cavity or space between them. 

 This space is the cavity of the primitive heart, the muscular 

 walls of which are derived from the splanchnic mesoblast, as 

 shown in Fig. 38A, ii. 



The heart is thus in the first instance a simple tube. This 

 soon becomes further differentiated, and bent on itself like an 

 S (see Fig. 37, ii. and iii.). The upper limb of the S forms the 

 atrium, which in the frog is early divided into two auricles by a 

 septum, the ventricle remaining undivided. In the fowl and 

 rabbit the ventricle is the first to be divided. 



