Chapter II 



EARLY AMPHIBIAN DEVELOPMENT: 

 A DESCRIPTIVE SKETCH 



§1 



It will be best to base the analytical treatment of development upon 

 a concrete example, and for this purpose the Amphibia are by far 

 the most suitable material, as analysis is much more complete in 

 them than in any other group of organisms. However, before em- 

 barking upon analysis, it will be desirable to give a brief descriptive 

 sketch of amphibian development in so far as it is relevant to sub- 

 sequent chapters ; to do this is the purpose of the present chapter. 



The chief stages of amphibian development are as follows : the 

 changes associated with fertilisation; cleavage, leading to the 

 blastula stage ; gastrulation, leading to the gastrula ; the elongation 

 of the embryo and the formation of the neural folds and tube, con- 

 stituting the neurula stage ; the appearance of the tail, and of the 

 remaining organ-rudiments, leading to the fully formed embryo, 

 which then hatches as a young larva ; and then the period of growth 

 and of functional differentiation. These stages overlap somewhat, 

 especially the last two, but they provide a useful broad classification. 



The typical amphibian egg is a spherical object of which one 

 hemisphere (known as the vegetative hemisphere) is loaded with 

 yolk, while the other hemisphere (the animal hemisphere) is freer 

 of yolk and contains the nucleus. There is, as a matter of fact, a 

 graded distribution of yolk from the animal to the vegetative pole. 

 In the Anura, the animal hemisphere is characterised by the posses- 

 sion of a layer of dark pigment at the surface, which distinguishes 

 it at a glance from the lighter-coloured vegetative hemisphere. A 

 similar distinction exists in the eggs of Urodela but is not so marked 

 because the pigment is less dark. Yolk being of a higher specific 

 gravity than the other constituents of the egg, it is found that after 

 the egg has been laid and fertilised and is free to rotate within its 

 membrane, the main egg-axis, or axis passing through the centres 

 or poles of both animal and vegetative hemispheres, is practically 



