Ill 



EXPERIMENTAL MODIFICATIONS 



421 



depends upon only one or several mechanisms. The investigated ionic inecjuilibria 

 are indeed related to cytolysis, but other factors may be at play. 



The reactivity of ectoblast to precytolytic agents already appears 20 hours 

 before gastrulation (Chuang Hsiao-Hui, 1955). This information might have some 

 bearing on explaining the results of experiments where the reactor is considerably 

 younger than the inductor, as those of Nieuwkoop (p. 402) and Johnen (p. 412). 



A notorious consequence of this research was a methodical investigation 

 (Holtfreter, 1944b, 1947b) of the ability of disintegrated germs or germ parts to 

 reorganize^. Even when thoroughly mixed, the cells of the various presumptive 

 territories seem able, as mentioned above, to reassociate according to their 

 nature. This is still true for neurula stages (Townes and Holtfreter, 1955), in- 

 cluding their neuro-epithelium. Thus, this layer has acquired the definite surface 

 organization which is necessary for selective cell adhesiveness (Fig. 12, pag. 321), 

 due probably to some correspondence or complementarity between the molecules 

 of the plasmolemma {cf. p. 317). It seems that during morphochoresis these prop- 

 erties gradually evolve by the acquisition of new devices for reciprocal linkage. 



It could however be erroneous to think that reaggregation amounts to a 

 regulation. The pattern of the reformed organs may on the contrary be very 

 abnormal. This is clearly shown by Fig. 81. The prosencephalic area of the neural 

 plate may reaggregate in some cases in a rather simple group of organs, in which, 

 however, the optic part is considerably exaggerated, reminding us of some of 

 Gallera's results (p. 372, Fig. 42). In other cases, brain parts, eye rudiments and 

 olfactory placodes may be scattered and multiplied in a most puzzling manner 

 (Boterenbrood, 1958). 



The relationship of dissociation and reaggregation to inductive capacity has 

 been investigated by Deuchar (1953). She used the three successive thirds of 

 archenteric roofs obtained from late gastrulae; in each piece of young ectoblast 

 she wrapped three reaggregated pieces from the same level joined together. The 

 results are presented in Table 5 in a way comparable to the controls already 

 used in Table 2 (p. 400). 



TABLE 5 



EFFECT OF DISSOCIATION, FOLLOWED BY REAGGREGATION, ON INDUCTION 



Region Total of Total Brain Eyes Neural Neuroid With 



experiments number of tube tail 



inductions outgrowth 



Contrary to what appeared in the controls (Table 2, p. 400) the numerical differences 

 are no more significant. A sort of uniformity has appeared, however, the anterior thirds 

 induce optic structures (which they did not in the controls) while the posterior pieces 



' See also Stearns (1955). 



Literature p. 48;^ 



