MAMMALIA 



449 



membranes — amnion and allantois — occur as among birds, but 

 their fate is somewhat different. The allantois typically fuses 

 with the outer layer of the amnion {false amnion Fig. 208, am^) 

 and the trophoblast (see above), and this combined tissue, the 

 chorion, becomes connected with the wall of the uterus by out- 

 growths or villi. These become closely associated with the 

 tissues of the mother. This combination of maternal and 

 embryonic tissues is called the placenta, and is the characteristic 

 organ of the Monodelphia or true mammals. 



It is by means of these united tissues that food and oxygen 

 pass from the blood of the mother into the blood of the embryo. 

 In the marsupials the attachment is very slight, and for this 



Fig. 237. Diagram of Segmentation of ovum in mammals. A, ovum; B, showing the early 

 differentiation into an outer layer which produces the trophoblast (see p. 447), and an inner mass 

 which produces the embryo; C, a later stage, ect., ectodermal portion of embryo; e«(., cells destined 

 to produce entoderm; in., inner mass of the cells which form the embryo; o, outer layer which 

 forms t. the trophoblast. 



Questions on the figures. — How does this differ from the segmentation in the 

 sea-urchin? What is the fate of the trophoblast? Examine reference texts and 

 learn how the ectoderm, entoderm, and mesoderm of the real embryo (the inner 

 mass of cells) are formed. 



reason uterine nutrition becomes insufficient relatively early 

 and the young must be provided for in some other way. The 

 marsupium, in which the milk glands open, presents the solution 

 of the problem of later development of the foetus. So at birth 

 the immature young of marsupials are placed by the mother in 

 the pouch. It is important to remember that the blood vessels 

 of the mother and the embryo are not continuous. The blood 



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