62 FERTILIZATION 



The weird case of the eggs of Fimdulus heteroclitus (Linn.) re- 

 quires re-investigation, particularly since Nakano (1953) found no 

 changes in Og uptake after fertilization of the eggs of Oryzias 

 latipes (Temminck & Schlegel). Very little respiratory increase 

 occurs in this egg until about two hours after fertilization. 



Whitaker (19336) has put forward the view that the post-ferti- 

 lization level of respiration, and therefore the change in respiration 

 at fertilization, is dependent on the pre-fertilization level, as many 

 fertilized eggs tend to have the same respiratory rate, — (i-2)fil02/ 

 hour/io/xl eggs at 21° C, while the pre-fertilization rate may be 

 well above or below this level. Whitaker has adduced convincing 

 evidence in favour of this contention, which Brachet supports in 

 Chemical Embryology. One might summarise Whitaker's hypo- 

 thesis by saying that the respiration of the unfertilized egg is 

 regulated by fertilization, many eggs which, before fertilization, 

 have widely different rates of O2 uptake, approaching the same 

 rate after fertilization. 



The idea that the past history of eggs and the females from which 

 they were obtained may influence the respiratory response to fer- 

 tilization was first put forward by Tyler & Humason (1937), 

 following their experiments on the eggs of Urechis caupo. Fertiliza- 

 tion of these eggs may produce an increase, a decrease, or no change 

 in O2 uptake, depending on how long the animals have been kept 

 in the aquarium tanks. When eggs are obtained from a female 

 which has not been kept long in the aquarium, the unfertilized 

 rate is relatively high; but in eggs from a female which has been 

 in the aquarium for a considerable time, and which is therefore 

 starved, the unfertilized rate is low. Kavanau (19546), in his 

 studies on the amino acid metabolism of sea-urchin eggs, claims 

 that the high respiratory rate of eggs freshly removed from the 

 ovary, and of oocytes, is due to the high energy requirements of 

 yolk protein synthesis, which he believes goes on at this time. 

 Kavanau's results are discussed in more detail in the next chapter, 

 but it seems doubtful whether this can be the whole explanation of 

 the wide variation in respiration observed in eggs of diftcrent 

 species. It is, however, clear that in future, more attention must 

 be paid to the history both of the eggs and of the females from 

 which they were obtained, before the experimental period. 



When the rate curves immediately after fertilization are ex- 

 amined, Fig. 13, it will be observed that, apart from any steady 



