H 



EARLY AMPHIBIAN DEVELOPMENT 



vertical. (This is the rule in the majority of the Amphibia, but it 

 should be mentioned that in Rana esculenta there are complications, 

 into which there is no need to go here, as a result of which the egg- 

 axis appears oblique. See Jenkinson, 1909 b.) 



Even before development can be said to have begun, therefore, 

 the tgg possesses one mark of dissimilarity between its various 

 regions, one mark of differentiation, which is expressed by saying 



Front i 



^^^^WTI^^*' 



^^<^re Hind ^^ 



Bac/f 



Fig. 2 



8cily 



Polarity and bilaterality in the frog's egg. Above, in equatorial view ; below, seen 

 from the vegetative pole : left, before fertilisation ; right, after fertilisation. The 

 unfertilised egg possesses a single main axis (polarity) at fertilisation, bilaterality 

 is established through the formation of the grey crescent in or close to the future 

 mid-dorsal line. (From Wells, Huxley and Wells, The Science of Life, London, 

 1929.) 



that the egg has polarity. This polarity is of great importance for 

 future development because the future front end of the animal will 

 be formed in proximity to the animal pole of the egg, and the hind 

 end of the animal close to the vegetative pole. Apart from this 

 polarity, which concerns the differential distribution of pigment, 

 yolk and cytoplasm, and the excentric position of the nucleus, the 

 egg is undifferentiated. 



