ion 



PRINCIPLES OF ANIMAL BIOLOGY 



viviparity, since only the spermatozoa arc shed into the water. The 

 sperms find the eggs, largely by chance, while the eggs are still in the 

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

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

 and presumal^ly contain all the necessary nourishment. 



Intermediate between ovoviviparoiis and viviparous forms are those in 

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



Fig. 149. — Uterus of mammal and contained embryo. The embryo is attached to 

 the uterine wall by an umbilical cord in which blood vessels belonging to the embryo extend. 

 These vessels do not connect with any vessels of the mother, but the capillaries of the two 

 systems approach one another very closely. F, fetal placenta; O, opening of oviduct; S, 

 maternal placenta. (Fro?n Kingsley's Vertebrate Zoology, Courtesy of Henry Holt and Co.) 



becomes attached to the body of the mother. Certain sharks (Fig. 150) 

 exemplify this intermediate condition. 



Summary of Locus and Nutrition of Embryo. — The variations in the 

 breeding habits of animals, with respect to the place of development of the 

 egg and the source of nutrition of the embryo, may be summarized as 

 follows : 



Ov^IPARITY 



(a) Eggs fertilized after being laid. 



(6) Eggs fertilized before being laid. 



Viviparity 



(a) Eggs retained until the yoimg are capable of independent exist- 



