Chapter III 



EARLY AMPHIBIAN DEVELOPMENT: 

 A PRELIMINARY EXPERIMENTAL ANALYSIS 



§1 



It has been shown that even before the amphibian egg is fertihsed 

 it possesses one differentiation, in respect of its egg-axis, which 

 determines the future positions of the anterior and posterior ends 

 of the embryo. The factors determining this axis of polarity must 

 be looked for at a stage before the egg is laid, for, while it is still in 

 the ovary, the yolk is already concentrated into one hemisphere. It 

 is possible that the orientation of the blood-vessels with regard to 

 the follicles and developing oocytes in the ovary may be the deter- 

 mining factor. It has been asserted^ that these blood-vessels are 

 so distributed that the arterial blood reaches the oocyte from one 

 side while the venous blood leaves it at the opposite side. This 

 w^ould cause a gradient in oxidation, and this in its turn would 

 produce a gradient in the relative amounts of cytoplasm and yolk, 

 more yolk being deposited in the regions of low oxidation. 



In this particular case, the matter cannot be regarded as certain, 

 since the same author has later qualified his assertion.'^ In other 

 organisms, however, it appears assured that the regions of the 

 oocyte where the rate of oxidation is highest will become the animal 

 pole of the egg and the anterior end of the embryo (see Chap. iv). 

 In the absence of evidence to the contrary, we are justified in 

 assuming that some causal agency of this type is operative in pro- 

 ducing the primary polarity of the amphibian egg. 



Once the amphibian egg is fully formed, however, gravity will 

 determine that the vegetative hemisphere (containing the relatively 

 heavy yolk) shall be undermost. This is normally brought about by 

 rotation of the egg within its membranes after being laid and 

 fertilised. But if the egg is forcibly inverted and maintained in that 

 position, gravity will determine that the yolk shall flow down to the 

 new lower surface. It does this by means of streaming movements, 



1 Bellamy, 19 19. ^ Bellamy, 1921. 



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