344 



GERMINAL ORGANIZATION INDUCTION PHENOMENA 



attained the pronuclei stage, there exists [a) the cortical field possibly reinforced by a 

 greater thinness of the pellicle (or coat) on the dorsal side^ (Dalcq and DoUander, 1948; 

 Dollander, 1953), and (b) a certain distribution of yolk in the endoplasm, not exactly a 

 true yolk gradient, but graded differences among the polar, equatorial and antipolar 

 zones. If interactions take place between these two sets of factors^, as was already suggested 

 for the earliest stages by the orientated rotations {cf. p. 324), the results would vary 

 according to the proportional value of the two involved sets of factors. It can be conceived 



Fig. 3 1 . Vertical section of an 

 inverted frog egg, during the 

 descent of the yolk. The egg 

 is now divided into two blasto- 

 meres. The platelets are indi- 

 cated by dots, heavier or lighter 

 according to their mean size. 

 The white part is the animal 

 cytoplasm. The yolk glides a- 

 long the cell walls from the veg- 

 etative (VEG) to the animal 

 pole (AN). From Pasteels, 

 1951- 



1 It has been recently assumed (Lovtrup, 1956) that the resulting facilitation of oxygen 

 supply to this presumptive dorsal region could explain its leading role in early develop- 

 ment; indeed, an experimental demonstration, however somewhat indirect, was given 

 that an unilateral restriction to the oxygen supply in fertilized eggs is able to cause a 

 displacement of the blastoporal groove (Lovtrup and Pigeon, 1958). On the other hand, 

 Dollander, (1958) has produced arguments in favor of a higher permeability in the region 

 of the "grey" crescent. This author has even shown that in eggs submitted to inversion, as 

 seen in Fig. 30, c, d, e, this property remains unchanged. 



2 The assumption that yolk is not only, as generally considered, nutritive material, bio- 

 logically inert, but plays a more dynamic role, is supported by two recent results. After 

 exploring the distribution of cathepsin, Deuchar concludes : "it has been specially interesting 

 to demonstrate that ventral tissues, which are so often placed at the bottom of gradients in 

 amphibian embryos that one has come to believe they are in some way inferior, have in fact 

 a higher catheptic activity for cell than dorsal tissues" (1958b, her p. 234). 



On the other hand, E.M. studies on chick blastoderm have shown that yolk platelets appar- 

 ently include, and later liberate, mitochondria, plus microparticles which could formRNA- 

 granules and circular bodies which could contribute to the endoplasmic reticulum (Bellairs, 

 1958). In Ranzi's laboratory Lanzavecchia and Coultre (1958) have made on frog embryos 

 E.M. observations suggesting the formation of mitochondria from yolk platelet material. 



