360 



GERMINAL ORGANIZATION INDUCTION PHENOMENA 



is apparent in the young epidermis, where they become slender, s-shaped and 

 quite numerous. 



A parallel approach was carried on in the laboratory of T. Yamada. The photographs just 

 published by Karasaki (ig59a, b) demonstrate considerable modifications, from the blastula 

 to the neurula stage, both in the cytoplasm and in the nucleus. Among the six or seven 

 plasmatic constituents which can be distinguished, the mitochondria show the most 

 conspicuous change. In the ectoblast they are ellipsoid and of a rather simple inner structure 

 and they do not change much in the epiblast. In the neurectoblast they are now found more 

 numerous, filamentous (3[J.), and often branched, with a highly complex inner structure. 

 The appearance of numerous ultramicrosomes around the vesicles of the endoplasmic 

 reticulum is in agreement with the increased RNA content. The changes in the nuclei 

 are the appearance of the nucleolus at the gastrula stage (in accordance with Brachet's 

 earlier descriptions), the gradual increase in density of the nucleoplasm and a more distinct 

 separation of the two layers forming the nuclear membrane. Various aspects revealing 

 cytonuclear interactions have also been described. 



Fig. 35. Effects of inductors labelled with glycine-2-*'*C. Autoradiographs, (a) Piece of 

 brain and eye embedded and introduced into the blastocoele. The induction is limited to 

 a thickening of the ectoblast. Note dense autoradiographs of graft and induction, and 

 faint autoradiographs of the internal endoblast. (b) Labelled piece of organizer used 

 in a sandwich. Note very dense autoradiographs on the graft, particularly on the nuclei. 

 The autoradiographs on the palisade are much stronger than on the epiblast in the upper 

 part of the figure. From Sirlin, Brahma and Waddington, 1956. 



Important biochemical changes have been progressively detected in the induced 

 neurectoblast, mostly thanks to advances in cytochemistry and in the use of isotopes. 

 When Brachet (1940, his p. igo) elucidated the meaning of the Unna procedure 

 (pyronine and methyl green) he immediately observed in the neurectoblast of 

 amphibian and bird embryo that induction was accompanied by an apparent 

 increase in ribonucleoproteins. This change remains one of the most characteristic 

 and general manifestations of primary — and also secondary — induction, and 

 reveals some change in intimate metabolism; the first signs of it can be detected 

 in the margino-dorsal region, and especially in the blastoporal lip. This early 

 change is quite apparent in the prechordal area of Xenopus (Pasteels, 1949). 

 Brachet also showed that the SH-protein complexes had roughly the same distri- 

 bution as the RNA. Fischer and Hartwig (1936) in their work on the speed of 



