222 



FERTILIZATION 



lized, when these degenerative (cytolytic) conditions are initiated, fail to 

 develop normally. If allowed to continue, cytolysis soon produces a condition 

 in which development is impossible, and dissolution of the egg results. 



3) Low Level of Respiration. While the egg is in this inhibited or arrested 

 state awaiting the event of fertilization, respiration is carried on at a steady 

 but low level. This respiratory level varies in different animal species (fig. 

 120). That this respiration rate may not be the direct cause of egg inhibition, 

 is shown by the fact that the rate of respiration does not always increase imme- 



FEMALE NUCLEUS 

 SPERM TAIL 



Fig. 115. (A) Early fertilized egg in upper Fallopian tube of the bitch (dog). Observe 

 the female nucleus before the first maturation division together with the sperm head 

 and tail. Note that the sperm, as in other mammals, enters the nuclear pole of the egg. 

 Observe further that the zona pellucida and the ooplasm are contiguous. (B) Section 

 of the egg of the dog, taken from the upper part of the Fallopian tube. Observe the 

 following features: (1) The sperm pronucleus is forming; (2) the egg nucleus has now 

 entered the meta^hase of the first maturation division; (3) the ooplasm of the egg has 

 shrunk away from the zona pellucida and a space is present between the egg and the 

 zona. This space is the perivitelline space, containing an ooplasmic exudate. (C) Sec- 

 tion of the egg in the Fallopian tube of the bitch, showing the formation of the first 

 polar body. 



diately following fertilization in all species (fig. 120). (Consult Brachet, J., 

 '50, p. 105.) Among the vertebrates, the low rate of oxygen consumption 

 of the unfertilized egg has been shown to continue for some time after fertili- 

 zation in the toad and frog egg and also in the egg of the teleost fish, Fundulus 

 heteroclitus. However, in the case of the egg of the lamprey the respiration 

 rate rises after fertilization (Brachet, J., '50, p. 108). 



4) Loss of Permeability. A final characteristic of the female gamete im- 

 mediately before fertilization is the loss of permeability of the egg surface 

 to various substances. Correlated with this fact is the presence of definite 

 ooplasmic or other egg membranes associated with the egg surface. The rela- 

 tionship between the ooplasmic surface of the egg and these membranes is 

 altered greatly after fertilization when the egg and the membranes tend to 

 separate. To what extent the loss of permeability of the egg surface is caused 

 by the intimate association of these membranes with the egg surface is prob- 

 lematical. The evidence to date suggests that under normal circumstances 



