250 CELLS, TISSUES, AND ORGANISMS 



among embiyologists for the fact that it inhibits the development of 

 chorda and produces strong microcephaly. I mean lithium ions, which 

 have been studied in detail, from the viewpoint of morphogenesis, by 

 Lehmann (1938), Pastcels (1954), and Hall (1942). It is now generally 

 admitted, as a consequence of their work, that lithium ions exert their 

 primary effect on the organizer itself, which shows reduced capacity 

 for induction. 



Cytochemical and biochemical studies made on lithium-treated 

 amphibian eggs have yielded a number of important results. First of 

 all, Ficq ( 1954b ) found that, in lithium-treated gastrulae, lithium ions 

 were accumulated by the dorsal half. More recent work by Dent and 

 Sheppard (1957) has largely confirmed this conclusion; they also ob- 

 served a strong accumulation of lithium in the medullary plate. Work 

 by Lallier ( 1954 ) and by Thomason ( 1957 ) has clearly shown that 

 lithium interferes with RNA distribution and synthesis in amphibian 

 eggs. According to Lallier, lithium decreases the RNA gradients, while 

 in Thomason's work this ion was found to inhibit markedly the in- 

 corporation of labeled phosphorus into the nucleoprotein fraction. 



We shall now consider the effects of physical treatments, such as 

 centrifugation, heating, or changing the pH of the surrounding medium, 

 on the RNA gradients. 



Centrifuging eggs during development is an easy way to modify 

 both the gradient distribution of substances or cell organelles and the 

 morphogenesis of the embryo. The most important experiments in this 

 field are those of Pasteels ( 1950, 1953 ) , who worked with amphibian 

 eggs. He found that centrifugation of freshly fertilized eggs led to the 

 formation of "hypomorph" embryos; they showed deficiencies in the 

 nervous system which might range from complete absence of the 

 system to strong microcephaly. When gastrulation of these centrifuged 

 embryos is normal, the result is the production of embryos which have 

 an almost normal tail but practically no head. On the other hand, 

 centrifugation of blastulae leads to the formation of double or even 

 triple embryos. 



Unpublished studies of Pasteels and Brachet have shown that the 

 centrifugation of both freshly fertilized eggs and blastulae produces 

 profound changes in the distribution of RNA. As shown in Figure 6, 

 ribonucleoproteins accumulate at the animal pole when fertilized, but 

 still uncleaved, eggs are centrifuged. If these eggs cleave normally, the 

 blastoporal lip forms in a normal position. But the material that in- 

 vaginates and corresponds to the organizer is much poorer in RNA 

 than is the organizer in normal eggs. This reduction in the RNA con- 

 tent of the invaginated material is accompanied by a marked decrease 

 in its inducing activity; the hypomorphoses described by Pasteels are 

 the logical result of such a situation. 



