ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 67 



of centrifugalisation. In eggs which contain a large quantity of 

 yolk {Crepidula, Styela, Rand) on the other hand, it seems that the 

 viscous cytoplasmic framework can be permanently distorted and 

 changed by displacement of the egg-contents as a result of pro- 

 longed centrifugalisation or inversion^ (see also pp. 94, 218, 313). 



§2 

 After the fixation of the axis of polarity, the most important deter- 

 mination in animals with bilateral symmetry is the determination of 

 the plane of the latter. In the frog, this is normally due to the point 

 of entry of the sperm. Before fertilisation, the ^gg is capable of 

 forming its plane of symmetry in any one of an infinite number of 

 possible planes passing through the egg-axis of polarity ; the actual 

 determination of a particular plane is fixed from the outside. The 

 matter has been considered in detail in Chap, iii (p. 36). We may 

 sum up our conclusions as follows. The machinery for realising full 

 normal bilateral symmetry is inherent in the egg ; even very slight 

 differential action of various external agencies can act as a trigger 

 permitting a particular plane of symmetry to realise itself: normally, 

 the entry of the sperm provides a strong diflPerential which readily 

 overrides the influence of other agencies. 



The formation of the grey crescent in the amphibian egg ap- 

 pears to be bound up with the establishment of an activity-gradient 

 of some sort extending dorso-ventrally across the equator of the 

 egg. The existence of this gradient is revealed by various facts. In 

 the first place, cleavage in the animal hemisphere proceeds more 

 rapidly in the dorsal meridian, so that at the close of segmentation 

 there is a slight gradient in cell-size from dorsal to ventral along 

 each circle of latitude. In the second place, there is the fact that 

 gastrulation and invagination is initiated in the dorsal lip region, 

 and then spreads progressively round each side until it reaches the 

 ventral meridian, and the blastopore lip becomes circular. 



Thirdly, there are the results of susceptibihty experiments. 

 These show that in anuran eggs exposed to lethal low temperatures 

 or lethal concentrations of KCN, NH4OH, HgCl2, and other toxic 

 agents, disintegration at any level begins at or near the dorsal 

 meridian, and extends thence round the egg towards the ventral 



^ Conklin, 1924. 



5-2 



