292 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



between the two cavernous bodies. At the anterior end of the penis the two 

 cavernous bodies diverge, forming the crura of the penis, which are attached to 

 the ischia. The cavernous bodies are spongy structures and in the sexual act 

 become distended with blood so that the penis is caused to project out of its 

 sheath, the prepuce. 



Draw the parts of the male genital system. 



Trace the rectum to the anus following directions given for the female. 



The structure of the testis may now be investigated. Each testis is inclosed 

 in a white fibrous sac, the parietal portion of the tunica vaginalis, which is in 

 reality the peritoneal pouch made by the descent of the testis (Fig. 65). Cut 

 open this sac, exposing the cavity in which the testis lies, this being a part of the 

 peritoneal cavity. The tunica vaginalis which lines this cavity is reflected over 

 the surface of the testis as the visceral portion of the tunica vaginalis, which forms 

 the outer thin coat of the testis. The point of deflection lies along the mid-dorsal 

 line of the scrotal sac, and a mesentery is formed along this line between the sac 

 and the testis. This mesentery is the mesorchium, corresponding to the broad 

 ligament of the uterus. The posterior end of the testis is a ttached to the posterior 

 wall of the scrotum by a short but stout ligament, the gubernaculum, continuous 

 with the mesorchium. The gubernaculum corresponds to the round ligament 

 of the uterus. The duct of the testis is the vas deferens or Wolffian duct. It lies 

 along the dorsal surface of the testis much coiled, the coiled portion being named 

 the epididymis. The epididymis begins at the anterior end of the testis as a 

 coil, the head of the epididymis. It then passes down the dorsal surface of the 

 testis as a coiled tube, the body of the epididymis. At the posterior end of 

 the testis it forms another coiled mass, the tail of the epididymis, to which the 

 gubernaculum is attached. From this the vas deferens proceeds anteriorly, 

 much convoluted, and passes into the inguinal canal where it becomes a straight 

 tube. The head of the epididymis is derived from the mesonephros and is 

 connected with the tubules of the testis by minute vasa ejferentia. The remainder 

 of the epididymis and the vas deferens are the Wolffian duct. 



Draw, showing contents of the scrotum. 



G. THE EMBRYONIC MEMBRANES 



i. General. There are four embryonic membranes in vertebrates. They are: the yolk 

 sac, the allantois, the amnion, and the chorion (Fig. 66). 



The yolk sac is simply an evagination from the ventral wall of the intestine. Its connec- 

 tion with the intestine forms a narrow yolk stalk. The yolk sac occurs in the embryos of all 

 vertebrates having meroblastic eggs and also in mammals, for they are descended from forms 

 which have meroblastic eggs. The yolk sac is filled with yolk, except in mammalian embryos 

 where it is empty. 



The allantois is a large evagination from the floor of the cloaca. It is primarily respira- 

 tory in function, but also probably serves to hold embryonic excretory materials. The adult 

 bladder develops at the base of the allantoic stalk. 



