Ill EXTRINSIC MORPHOCHORESIS 36 1 



reduction of brillant cresyl blue found that the dye was most rapidly reduced in the 

 neural plate. The same high activity was observed in the presumptive neural 

 territory and, still earlier, in the dorso-marginal zone of earlier stages (Piepho, 

 1938). 



Another body of significant data concerning the RNA has just been published 

 by Rounds and Flickinger (1958). Using frog embryos at six stages, from the 

 early gastrula to the neurula with closing tube (Shumway's stages 10 to 15, 

 Fig. 36, A), they chilled them and cleanly separated the inducing and reacting 

 material. The pooled fragments from a dozen embryos were homogenized 

 and submitted to successive centrifugations with appropriate saline dilution. 

 Three fractions were obtained and precipitated by trichloroacetic acid in order 

 to estimate together RNA, DNA and protein nitrogen content in: (a) the 

 soluble fraction, containing proteins soluble in such media, lipids and some of 

 the microsomes; (b) the yolk fraction, formed of the dissolved yolk platelets; 

 {c) the residual fraction made up of the insoluble cellular debris, such as fragments 

 of cell membranes, pigment granules, mitochondria and some of the microsomes. 

 The method is thus characterised by measuring protein and RNA relatively to 

 the DNA content, which affords a cellular basis of reference. 



The protein nitrogen content is constant for fractions a and c both in inductor 

 and reactor, but for fraction b there is a decrease in the inductor during its 

 invagination and "the rate of decrease becomes pronounced just prior to the 

 appearance of the neural plate" (Fig. 36, B). In the reactor the same fraction 

 is relatively constant with the exception of a slight decrease at the time when the 

 cells of the neural tube are beginning to differentiate (Fig. 36, C). 



The RNA content of fraction a remains constant both in inductor and in 

 reactor. For fraction b, a regular decrease is observed in the inductor, while in 

 the reactor a fall only occurs, as for protein, in the plain neurula. For fraction c, 

 the RNA level of the inductor increases slightly during invagination, then de- 

 creases significantly (6 consistent determinations) during the most active induction 

 period, to increase again while the neural plate is appearing and soon closing. 

 In the reactor, at the same time, a great increase is observed (Fig. 36, D, E). 

 The total levels of RNA in the inductor and reactor (Fig. 36, F) are not less 

 impressive. Clearly, the amount of this substance is first reduced in the inductor 

 just when the archenteric roof becomes subjacent to the ectoblast. Then an 

 increase takes place in both layers, more intensely in the reacting ectoblast. 



These data suggest that yolk platelets progressively break down in the chordo- 

 mesoblast during invagination with a subsequent release of RNA to the cyto- 

 plasmic granules. The RNA is then used in some way during the incipient period 

 of induction, but is apparently not immediately recuperated, at least not as such, 

 by the reacting ectoblast. As soon as morphological changes are perceptible in 

 the neural plate and in the underlying archenteric roof the level of RNA increases 

 in both layers, however not at the same rate, but distinctly more in the ectoblast. 

 This bulk of new information is especially valuable. 



Important cytological observations have also been made by means of the isotopes. 

 In fact, they have been obtained by operative techniques (Eakin et al., 1951 ; Ficq, 

 1954b; Sirlin, Brahma and Waddington, 1956), but the interest of their results Hes 



Literature p. 483 



