Ill 



EXPERIMENTAL MODIFICATIONS 



415 



Thirdly, the cause of the aberrant morphochoresis hes in the collapse oi" the blastocoele 

 roof (Fig. 75-77)- At the susceptible stages, the rumpling and folding of this region recedes 

 only slowly, and causes a distinct increase of true basophily (Fig. 78, a, b, c, d). During 

 gastrulation, a pancake-shaped mass of tissue is found there (Fig. 78, b) which finally splits 

 into a mesoblastic basophilic thickening and a more moderate thickening of the epiblast 

 (Fig. 78, c). Neurogenic induction soon begins but is somewhat retarded relative to the 

 main system (Fig. 78, d). 



Fig. 76. Immediate effect of centrifugation on the frog blastula. The roof of the blastocoele 

 is deeply folded. From Pasteels, 1953. 



Fig. 77. Section of the collapsed roof of the blastocoele seen in Fig. 76. Only parts of the 

 cavity remain free (bl.). From Pasteels, 1953. 



It is impossible to escape the idea that local enrichment in ribose nucleic acid 

 (free or combined with a protein), as a consequence of a mild lesion in the 

 collapsed ectoblast, is responsible for secondary morphochoresis. If the treatment 

 is applied at non-susceptible stages, the collapsing is transitory, the tissue unfolds, 

 and the effect is deleted, which shows the role of the mechanical consequences of 

 the centrifugation and the way they affect the ectoblast. This is important for 

 it has been claimed that Karasaki and Yamada (1955) repeated and confirmed 

 Pasteels' experiment, but did not obtain any result when using a piece of ectoblast 

 isolated before centrifugation. No reaction whatever was obtained and therefore 

 some authors were tempted to dispose of Pasteels' results by assuming that they 

 were due to induction by the yolk mass and the inner marginal zone with which 

 the ectoblast comes into contact (Toivonen, 1954b). 



Lileralure p. 483 



