88 PRINCIPLES OF EMBRYOLOGY 



evidence, suggests that the gradients are fundamentally based on the rates 

 of certain critical chemical reactions. Moreover, if young embryos are 

 constricted, by being tied in a loop of hair, the degree to which the animal 

 and vegetative gradients interact with one another depends on the width 

 of the comiection between the two halves, which again suggests that 

 diffusmg chemical substances are involved. 



We still have little idea what these substances are. Very careful measure- 

 ments have failed to reveal any difference in respiration between the 

 animal and vegetable halves. However, Child (1936) observed some 

 years ago that by the blastula stage there is a double gradient in the rate 

 of reduction of vital dyes such as Janus Green and methylene blue, which 

 are indicators of redox-potential. There is a high level of reduction acti- 

 vity at the vegetative pole, falling off towards the animal, and simul- 

 taneously a weaker gradient running in the opposite direction from the 

 animal pole towards the vegetative. Horstadius (1952) has recently studied 

 these gradients in isolated animal and vegetative halves, and in animal 

 halves into which micromeres have been implanted, and has shown that 

 their behaviour parallels that of the postulated gradients of animal- 

 vegetative tendencies. The biochemical meaning of the gradients in dye 

 reduction is not yet understood, nor can one be certain whether they are 

 related to the causes of the animal and vegetative gradients or are merely 

 among their effects (Fig. 5.5). 



A perhaps more promising clue to the processes underlying the grad- 

 ients is the fact that in rather late cleavage stages the vegetative region of 

 the egg comes to require the presence of sulphate ions in the medium, and 

 its development is inhibited in artificial seawaters from which they are 

 absent. Lindahl (1942) suggests, on these and other grounds, that the 

 reactions proceeding in the vegetative region give rise to aromatic waste 

 products (formed from protein catabohsm) which are normally disposed 

 of by being combined with sulphate. Horstadius and Gustafson (1954) 

 have studied the animahsing and vegetativising effects of various amino- 

 acids as well as substances which might be expected to play a role 

 in carbohydrate metabolism. The latter group were, on the whole, animal- 

 ising, which supports the suggestion that the animal tendencies are con- 

 nected with carbohydrate metabolism; but the effect of the amino-acids, 

 although usually towards vegetativisation, was not invariably so, and 

 further work will be necessary before the experiments can be fully 

 interpreted. 



Meanwhile there is another quite different theory in the field. Ranzi 

 (195 1) presents evidence to show that lithium and thiocyanate have im- 

 portant effects on the viscosity of solutions containing elongated protein 



