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HARRISON'S STAGES 



Figure 3. The left hand figure shows the (A) typical cross section of the neurula 

 with the ectodermal subdivisions marked, the (B) is a diagram of the left side showing 

 the subdivisions of (A) reconstructed as lunes on the lateral surface. The right hand 

 figure shows Gillette's graphs of the cellular redistribution of the ectodermal cells during 

 neural plate formation. 



in the various parts of the gastrula to be of secondary importance in the 

 inductive action. 



Brachet ('38) in cyto- and histochemical studies localized the SH 

 groups in various stages of development. With the exception of the two 

 cell stage, the SH groups are predominantly situated in the animal re- 

 gion of the embryo and during gastrulation are strongly centralized 

 around the dorsal lip, after which there is a heavy localization of these 

 substances in the nervous system. In his 1944 presentation of the vari- 

 ous lines of chemical investigation, Brachet shows that the content of 

 ribonucleic acid decreases in the dorsal lip during its invagination (cf. 

 Fig. 94 in Brachet ('44) ). There is a coordinate change in the distribu- 

 tion of the ribonucleoproteins and a marked shift in their localization 

 during neurulation. This is coincident with the pattern as shown in his 

 Figure 92. 



