GRADIENT-FIELDS 49 



embryo, and the direction of its plane of symmetry depend on 

 the interaction of two factors: (1) the yolk gradient, localised 

 in the internal cytoplasm, and (2) a gradient-field in the cortex, 

 the so-called "cortical field". The variable quantity in this 

 field is a hypothetical "C-factor", whose nature is yet unknown. 

 In normal eggs, its maximum intensity coincides with the centre 

 of the grey crescent; its minimum lies at the opposite side. 

 All gradients in this field run therefore in a dorsoventral 

 direction, and the planes of equal intensity intersect the egg 

 surface in a series of concentric circles. This field originates 

 during the formation of the grey crescent, which, according to 

 the work of Ancel and Vintemberger, is accompanied by a 

 contraction of the egg cortex towards the dorsal side (see 

 above, p. 41). 



In sea urchins, cortical factors apparently also play a role. 

 Pease (1939) studied eggs of Dendraster which had been 

 centrifuged at a very high speed. He found that the position 

 of the ventral side of the embryo was determined by the inter- 

 action of a cortical gradient system with certain substances 

 localized in the internal protoplasm, which accumulate at the 

 centripetal side during centrifuging. This case, therefore, is 

 completely analogous to that of the amphibians discussed above. 



Summarizing these facts, we can state with some confidence 

 that the directional organisation of the egg is governed by the 

 interaction of axial and cortical gradient- fields. These fields 

 determine the polarity and symmetry of the egg, and thereby 

 simultaneously determine the main directions of the future 

 embryo. This directional organisation of the egg provides it 

 with what may be called a fixed system of coordinates, and 

 all further developmental processes are oriented with regard 

 to this system. There are fixed relationships, therefore, between 

 the polarity and symmetry of the egg, and the organisation 

 of the future embryo. 



Raven - Outline Physiologie 



