294 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



umbilical veins are the abdominal veins of lower forms. The amnion and the chorion never 

 contain any blood vessels. 



The yolk sac may occur in the embryos of any group of vertebrates. The allantois, 

 amnion, and chorion occur only in reptiles, birds, and mammals, which are hence designated 

 amniotes. The allantois spreads out inside of the chorion and becomes fused to the chorion 

 forming a chorio-allantoic membrane (Fig. 66D). In the placental mammals this chorio- 

 allantoic membrane comes into close contact with the internal wall of the uterus and in the 

 highest mammals fuses inseparably with the uterine wall (Fig. 66E). The compound structure 

 thus produced by approximation or fusion of the chorio-allantoic membrane with the uterine 

 wall is designated the placenta. It is of various shapes in various mammals, the name placenta 

 being derived from the disk shape of the human placenta. 



2. Anamniote embryo of the dogfish. Cut open the pregnant uterus of a 

 dogfish and remove an embryo or examine embryos provided. Note that the 

 embryo is naked. From the middle of its ventral wall hangs the large yolk sac, 

 filled with yolk and attached to the body by the narrowed yolk stalk. The yolk 

 sac is covered externally by a layer of the body wall and internally consists of 

 the intestinal wall inclosing the yolk. As the yolk is used up the yolk sac is 

 gradually drawn into the body. Draw. 



3. Amniote embryo of the cat. If pregnant females are available, open one of 

 the enlargements in the horns of the uterus. The enlargement contains an 

 embryo. Note that the embryo is inclosed in a thin membrane, the amnion. 

 On the inner surface of the uterine wall at the enlargement note a thickened 

 vascular ring of tissue. This is the placenta; it will probably peel off from 

 the uterine wall, especially in advanced stages of pregnancy. Cut into the 

 amnion and note the umbilical cord extending from the ventral side of the 

 abdomen of the embryo to the inner surface of the amnion. The umbilical cord 

 is a connection between the embryo and its own membranes and not, as popularly 

 supposed, a connection between the embryo and the mother. There is no 

 direct connection between embryo and mother. 



H. SUMMARY OF THE UROGENITAL SYSTEM 



1 . The urogenital system is derived from the mesomere of the embryo. 



2. The urinary or excretory system consists of the paired kidneys and their ducts. The 

 kidneys are composed of tubules opening at one end into the coelom by a nephrostome and at 

 the other end into the collecting duct. 



3. In the evolution of the vertebrates there have been three successive kidneys, each 

 situated more posteriorly than its predecessor. 



4. The first kidney of vertebrates is called the pronephros. Its duct is the pronephric 

 duct. It appears in the embryos of all vertebrates but functions in the adults of only cyclo- 

 stomes and a few fishes. 



5. The second kidney is the mesonephros or Wolfnan body. Its duct is the mesonephric 

 or Wolffian duct; this is a continuation of the pronephric duct. It enters the cloaca. The 

 mesonephros is the functional kidney of most adult fishes and all Amphibia. 



6. The third kidney is the metanephros. Its duct is the metanephric duct or ureter. 

 The ureter and part of the metanephros arise by evagination from the Wolfnan duct. The 

 aietanephros is the functional kidney of the adults of all the amniotes. 



