CHAPTER XL VI. 



MAMMALIAN EMBRYOLOGY 



In both the chick and the frog the two forms we have thus far dis- 

 cussed the eggs have passed out of the body of the mother. In the 

 frog, the entire embryo developed after the egg left the mother, while 

 in the chick where fertilization is internal, development began before 

 the shell was formed, so that an embryo, approximately twenty-four 

 hours old, was already present when the egg was laid. 



Now we shall deal with viviparous animals, that is, with those which 

 give birth to living young. 



It will be understood quite readily that in those cases where living 

 young are brought forth, the development must take place within the 

 body of the mother, but, even in viviparous animals there are sub- 

 divisions. One sub-division is made up of such animals as the duckbill, 

 the Australian ant-eater, the Australian kangaroo, and the American 

 opossum. In these animals, known as Marsupials, the female bears a 

 pouch in the abdominal region in which the young are placed at a very 

 premature age. In fact, in the opossum, the embryo may be only about 

 eight days of age when it is born for the first time so to speak. The 

 mother then places it within the pouch or marsupium, and here the young 

 continue their development until able to lead an independent existence. 



In all the higher forms of mammals with which the student is 

 familiar, fertilization is internal, as in the chick, and the embryo passes 

 through a process similar to that of the chick, except that this embryonic 

 process takes place within the mother's body. 



There is, however, in viviparous animals no real yolk-supply as in 

 both the chick and the frog egg. Consequently, there must be some 

 kind of an arrangement by which the young not only become attached 

 to the uterus of the mother, but there must also be an arrangement by 

 which a blood-supply can pass from mother to offspring, thus taking 

 the place of the nourishment the yolk furnishes in egg-laying animals. 



The mammalian egg, not possessing a yolk, is very small. 



The original development of egg and sperm, however, is not very 

 dissimilar to that already described for the chick. 



Before entering into the study of mammalian embryology proper, 

 it is well, at this point, to understand the terminology usually applied to 

 the life-history of a mammal. 



First, the period of gestation or true embryonic period. It is during 

 this time that the embryo depends upon its connection with the mother's 

 uterus for nourishment. Gestation extends from the time of the fertili- 

 zation of the egg to the time of birth. 



