EXPERIMENTAL MODIFICATIONS 69 



hormones (stilboestrol, oestradiol, testosterone), which have been 

 studied by Tondury (1947), Cagianut (1949) and Rickenbacher 

 (1956). These substances inhibit cleavage or make it abnormal. 

 Furthermore, they also modify the normal gradient of RNA distri- 

 bution. It seems that, owing to alterations of the mitotic apparatus 

 during early cleavage, RNA becomes unevenly distributed in 

 daughter cells. At later stages, strong abnormalities of develop- 

 ment can be found, the most conspicuous being unequal differen- 

 tiation of the medullary folds, which results in an asymmetry of the 

 nervous system. It is a very interesting fact, which certainly deserves 

 much more extensive study, that, according to Cagianut (1949), 

 addition of yeast RNA to embryos which have been treated with 

 steroid hormones definitely improves their differentiation. 



Something should be said about another chemical, which is 

 famous among embryologists for the fact that it inhibits the develop- 

 ment of chorda and produces strong microcephaly. This is the lithium 

 ion, which has been studied in detail from the viewpoint of mor- 

 phogenesis by Lehmann (1938), Pasteels (1945) and Hall (1942). 

 As a consequence of their work, it is now generally admitted that 

 lithium ions exert their primary effect on the organizer itself, which 

 shows a reduced capacity for induction. The competence of the 

 ectoblast, i.e. its capacity to react to the inducing stimuli emanating 

 from a normal organizer, are well known to all embryologists. 

 The most spectacular effects of lithium are the total absence of the 

 chorda (the somites having fused together under the neurula tube) 

 and microcephaly which can be so marked as to lead to cyclopia. 



Cytochemical and biochemical studies made on lithium-treated 

 amphibian eggs have yielded a number of important results. First 

 of all, Ficq (1954b) used a very original autoradiography method, 

 based on the nuclear reactions undergone by lithium when it is 

 placed in a neutron flux, to detect lithium in early gastrulae. She 

 found that in lithium-treated gastrulae lithium ions are accumulated 

 by the dorsal half. More recent work by Dent and Sheppard (1957) 

 has largely confirmed Ficq's pioneer experiments of 1954. Strong 

 accumulation of lithium in the medullary plate was also observed 

 by these authors. 



References p. 90/93 



