Ill SECONDARY AND MINOR INDUCTIONS 467 



How and why this ridge is formed, and receives the phosphatase-loaded primary 

 gonocytes is partly known, and probably also implies induction. Indeed, the 

 somatopleural epithelium is activated along the root of the dorsal meso of the gut 

 by two different influences : anteriorly, by the migration of a massive cord of 

 neural crest cells, yielding the "medullary" part of the adrenal complex (not 

 unified in amphibians) ; and more posteriorly, by the arrival of the gonocytes 

 (Fig. 98), which cause the proliferation of the coelomic epithelium known as the 

 cortex of the gonad. 



At this level, the attraction of some of the urogenital cords is possibly due to 

 another induction, but more certainly the penetration of these elements into the 

 forming gonad exerts an inductive action on the cortex. This results from the 

 experimental analysis performed in Gallien's laboratory [cf. 1954, 1956, 1958), 

 especially by Houillon (1957) on Pleurodele larvae. The chain of events can be 

 cut at its root by hindering the backward elongation of the pronephric duct. If 

 the mesonephric blastema remains quiescent, no presumptive medulla cells enter 

 the tiny cavity of the genital ridge and the presumptive cortex also remains 

 quiescent. 



At this stage, the genetic constellation of factors enters the scene, and its inter- 

 vention is as decisive as brief. 



If the combination of chromosomes is 9 (zw in amphibians), the cortex proliferates and 

 organizes into an inner-secretory gland which rapidly influences the whole soma. From 

 what is known of the process in fish and birds (p. 315) it can be admitted by interpolation 

 that the liver of the young g elaborates vitellins and ribonucleoproteins which allow the 

 gonocytes to grow intensely and to perform vitellogenesis. 



If the chromosomal combination is d (zz in amphibians), the medulla is more activated 

 than the cortex and mobilizes the gonocytes to be colonized. This process seems to imply 

 a modification of the surface properties and activities of the gonocytes, which may be rather 

 similar to an induction. The rudiment of the male gonad, enclosed by the quiescent cortex, 

 begins to produce the corresponding sexual hormones, and, sooner or later, the Wolffian 

 and Mullerian duct will be influenced by these hormones. The liver is less sollicitated, at 

 least not in the same sense as in 9 embryos, growth of the auxocytes remains limited, and 

 yolk is completely lacking. 



When sexual hormones are administred at the appropriate period and in due quantity, 

 they can reverse the balance between cortex and medulla. It seems that they act by a 

 differential action, depressing one part in favor of the other. However, it is not excluded 

 that they might awaken some inductive processes, like the drawing into the medulla of the 

 gonocytes, but definite information is lacking on this point. 



Considering these events, it appears that the balance of the genes acts clearly 

 on the hormonal setting, but less explicitly on inductive processes. In the organo- 

 genesis of the gonads, which, so to speak, closes the circle and achieves the work 

 of the morphochoretic mechanisms, induction seems indifferent to the chromosomal 

 setting, as in all preceding phases, and apparently only needs healthy nuclei. 



F. General remarks on induction 



After this rather complete — though not exhaustive — survey of our up-to-date 

 knowledge of induction processes, we should try to take a not too distorted birds- 

 eye view of them, to see how thoroughly we understand their general picture. 



Literature p. 483 



