I06 HOW ANIMALS DEVELOP 



maternal blood circulations into close contact. In 

 some other animals, for instance, some kinds of fish, 

 the embryo develops for some time within its 

 mother's body, but it is only in the mammals that 

 the connection of the two blood streams is at all 

 intimate. Even in mammals maternal blood never 

 circulates actually through the embryonic blood- 

 vessels. If it did, not only might the mother's female 

 sex hormones disturb the development of the sex of 

 her sons, but the embryo would actually be killed by 

 the foreign proteins, since each animal has a chemical 

 individuality which has to be respected. There is 

 therefore always a filter between the maternal blood 

 and the embryo. This filter is the placenta, which is 

 developed from the trophoblast, or non-embryonic 

 part of the embryo which was described above. The 

 trophoblast lies against the wall of the uterus, and 

 substances which pass from the mother into the 

 embryo have to pass through the lining of the 

 uterine blood-vessel, through the connective tissue 

 surrounding it, through the wall of the uterus, then 

 into the embryonic circulation through the wall of 

 the placenta, through connective tissue, and finally 

 the lining of the embryonic blood-vessel. There are 

 thus six layers to be passed through (Fig. 26, «). But 

 these conditions only hold in the most primitive 

 types of placenta ; they are found, for instance, in 

 the pig and the cow. In some other mammals the 

 passage of substances is made easier by the disap- 

 pearance of some or all of the maternal layers. In 

 the 5tag the maternal wall of the uterus disappears 



