320 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



head ; on the other hand it may be that the individuative component 

 of this action is really due to the apico-basal individuation-field, 

 and that the organiser region here again only exerts a releasing 

 action, whether by the diffusion of chemical substances, or by 

 neuroid transmission, or by other means. 



It is further probable that the dorso-ventral gradient contributes 

 to the total individuation-field of the embryo, e.g. by introducing 

 a dorso-ventral polarity (see p. 357 for the dorso-ventral polarity 

 of limb-areas). 



Per contra^ the primary individuation-field also exerts indirect 

 effects owing to the graded accumulation of yolk, cytoplasm, fat 

 and other substances along its axis (p. 311). This has secondary 

 effects upon the rate of cleavage and relative cell-size in different 

 parts, which are of importance in the mechanics of gastrulation ; 

 and also upon the amount of raw materials available in different 

 parts of the body. The apical region of the primary field will always 

 attempt to form a brain : but it can only form a brain of normal type 

 if it contains less than a certain proportion of yolk, and less than a 

 certain proportion of fat. Thus the indirect effects of the primary 

 gradient are adjusted to co-operate with the direct effects. 



The position of the grey crescent itself is a prior example of such 

 interaction. The point of sperm-entry decides the meridian of the 

 grey crescent, and therefore the meridian on which the high point 

 of the secondary gradient will lie. However, the precise latitudinal 

 position of this high point is not sharply predetermined at a fixed 

 level, but depends upon conditions in the primary gradient and can 

 be experimentally modified by modifying these. For instance, ex- 

 posure of the frog's egg to depressant agencies (e.g. Njio LiCl) 

 during early segmentation leads to the dorsal lip being formed 

 nearer the animal pole than usual; in some cases even above the 

 equator (fig. 149).^ Temperature gradients (p. 339) applied during 

 segmentation also influence the position of the dorsal lip.^ 



The same sort of interaction of two gradient-systems occurs in 

 the Echinoderms, only the high point of the secondary gradient is 

 here directly vegetative instead of dorsal. In all probability, similar 

 processes are at work in Annelids and Arthropods (see p. 309). 



^ Bellamy, 1919. 



^ Dean, Shaw, and Tazelaar, 1928. 



