184 



PRINCIPLES OF ANIMAL BIOLOGY 



umbilical vessels) are bathed, as shown by the diagram at the right in 

 Fig. 153. The physiological operation of the two blood systems is 

 precisely like that of blood and adjoining tissue. Digested food and 

 oxygen in the maternal blood are transferred to the fetal blood, because 

 they are at higher pressure in the former. Accumulated urea and carbon 

 dioxide go in the opposite direction because they are at greater pressure 

 in the embryo. The fetus is thus being fed, and its wastes removed, 

 as efficiently as if it were a rapidly growing tissue in the mother's own 

 body. No blood cells are transferred in either direction, however; the 

 exchange is entirely a process of diffusion and osmosis. 



Forms in which the embryo is connected with the maternal uterus 

 by a placenta are spoken of as truly viviparous. Hydra and some of 



MATERNAL 



LAKE OF MA 

 TEPNALJLOOp 



WALLOP 

 UTERUS 

 (PART) 



FETAL 

 BLOOD 

 VESSELS 



UMBILICAL 



ARTERY AND 



VEIN, 



Fig. 153. — Position of fetus in uterus (left), with its attachment by umbilical cord and 

 placenta to uterine wall: F, fetal placenta; 0, opening of oviduct; <S', maternal placenta. 

 Rectangle shows approximate location of dissection of human placenta at right. {Left 

 after Kingsley, " Vertebrate Zoology," Henry Holt and Company, Inc.) 



the jellyfishes, among aquatic animals, exhibit something like viviparity, 

 since only the spermatozoa are shed into the water. The spermatozoa 

 in these forms find the eggs, largely by chance, while the eggs are still in 

 the maternal ovary and penetrate the eggs in that situation, and the fer- 

 tilized eggs develop there for a time. In these cases the eggs are large 

 and presumably contain much of the necessary nourishment. 



Intermediate between ovoviviparous and vivipai-ous forms are those 

 in which the young develops for a considerable time in the egg and later 

 becomes attached to the body of the mother. C^ertain sharks (Fig. 154) 

 exemplify this intermediate condition. The expanded end of the yolk 

 sac becomes attached to the wall of the uterus, terming an organ like 

 the placenta of mammals. The young receive nourishment through 

 it from the mother. 



