XIX. THE DEVELOPMENT OF MAMMALS 
A. DEMONSTRATION OF THE GRAVID UTERUS IN 
RODENTS AND CARNIVORES. 
i» General Examination (guinea pig, rabbit, cat, preferably 
fresh specimens).—Note the greatly enlarged veins and arteries 
indicating a corresponding increased blood supply; the number of 
embryos present, their size, and their location in the uterus. 
Draw a view of the uterus showing the number and location of the 
embryos. 
2. Extraembryonal Structures and Their Relation to the 
Embryo. —After carefully opening the uterus in the region of one 
of the embryos, identify (a) the chorion, a sac which encloses the 
embryo and is closely applied to the lining of the uterus, from 
which it can, however, be separated in most cases, in toto; ( b ) the 
placenta, either discoiclal or zonary, a vascular structure attached 
to the inner wall of the uterus, formed in part from its lining 
mucous membrane and in part from a differentiated region of the 
chorion, and hence supplied with two sets of blood channels, one 
connected with the maternal and the other with the embryonal 
circulatory system, (c) the umbilical cord passing from the pla¬ 
centa to the embryo and containing the veins and arteries which 
connect the placenta with the circulatory system of the embryo, 
and (d) the amnion, a thin transparent membrane which forms a 
sac enclosing the embryo and containing the amniotic fluid. 
Draw one or more diagrams showing the relation of the embryo to the 
extraembryonal parts. 
B. DEMONSTRATION OF LACTATION. 
In the various specimens of the gravid female guinea pigs 
studied note the condition of the mammary glands, and their 
increasing size with the advance of pregnancy. Examine a fresh 
dissection of a specimen in lactation (after the birth of the young) 
and demonstrate by an incision into the gland tissue, the presence 
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