FERTILISATION 



49 



Considerable controversy has raged about whether the entry of the sperm 

 fixes the position of the dorso-ventral plane, or whether this is already 

 determined in the egg and some mechanism ensures that the sperm always 

 enters on it. The evidence is still somewhat conflicting, but seems to 

 indicate that there is something in both ideas. It is certainly true that if 

 eggs (e.g. of frogs) are artificially fertilised by sperm placed on the surface, 

 the point of entry of the sperm determines the plane of bilateral symmetry; 



mocrom. 



Figure 3.3. 



Movements of ooplasms following fertilisation in Limnea. {a) Shows the 

 vegetative plasm (dots) before fertilisation. The entry of the sperm is fol- 

 lowed [h) by a movement of this plasm towards the animal pole. A little later 

 (c) the nuclei (pron.) conjugate near the animal pole and a new ooplasm (close 

 dots) appears there. During the cleavage divisions, this extends towards the 

 vegetative pole {d and c), the original vegetative plasm becoming less easily 

 recognisable. (After Raven 1948.) 



but it is also probable that there is a predisposition of a fixed plane in the 

 egg, which controls the point of entry in normal unforced fertilisation. 

 (For a full account of the rather comphcated events following fertilisation 

 in frogs, see Ancel and Vintemberger 1948 and p. 146.) 



Accompanying the visible structural changes produced by fertiHsation, 

 there are almost certainly associated alterations in the biochemical pro- 

 cesses proceeding in the egg. Many years ago Warburg showed that the 

 oxygen uptake of fertihsed sea-urchin eggs is very considerably higher 



