ZOOLOGY. 



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. 205, am 2 ) 

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

 chorion, becomes connected with the wall of the uterus by 

 outgrowths or villi. These become closely associated with the 



t 



in. 



FIG. 234. Diagram of Segmentation of ovum in Mammals. A, ovum; B, showing 

 the early differentiation into an outer layer which produces the trophoblast (see p. 437), 

 and an inner mass which produces the embryo; C, a later stage, ect., ectodermal por- 

 tion of embryo; ent., 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 segmenta- 

 tion 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. 



tissues of the mothers. This combination of maternal and 

 embryonic tissues is called the placenta, and is the character- 

 istic organ of the Placentalia 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 em- 

 bryo. In the marsupials the attachment is very slight, and 

 for this 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 



