EXPERIMENTAL MODIFICATIONS 



41 I 



C. Experimental modifications of morphochoresis 



Many of the possible ways to modify morphochoresis have been used on the 

 whole egg, on isolated parts of the marginal zone, or on the ectoblast before it 

 normally reacts. It is not necessary here to consider separately general treatments 

 on the germinal system, transplantations, and explantations; however, the 

 distinction between physical and chemical procedures will be helpful. 



(fl) Physical means 



The three accessible properties of physical systems — distribution in space, 

 modification by time, and energetic changes by temperature or radiations — have 

 been explored. 



The influence of age on the reacting ability of the ectoblast has been recognized 

 by Holtfreter (1938a); however, he thought, wrongly, that the ability to form a 

 l^rain disappeared first, and was practically suppressed when the explants have 

 been cultivated during the period of time necessary for the controls to attain the 

 blastopore stage. Gallera (1952) using newt gastrulae and a technique (Fig. 72) 



Fig. 72. Technique for testing the maintenance of competence of isolated ectoblast. (a) 

 Two early gastrulae have received a large mark of Nile blue, (b) Their ectoblastic areas 

 are then dissected and combined with their inner sides together, (c) After due cultivation, 

 a piece of this vesicle is used (d) to replace the anterior part of the neural field in a young 



neurula. From Gallera, 1952. 



avoiding the unfavorable action of the saline medium on the inner side of the 

 explant, has been able to maintain brain competence until a stage corresponding 

 to incipient neurulation in the control. Just a few hours later, when a pigmented 

 hem marks the limit of the plate, the competence for acrencephalon inductions 

 briskly disappears, while balancers and stomodaeum are still produced. During 

 the period of declining competence, the ability to form optic material, often 

 only one vesicular expansion, is more resistant than telencephalon production. 

 This result again shows how important is the technique used to determine the 

 stage at which a change of potency occurs in a given layer or territory. Let us 

 remember that with the fold technique the ability to form neural structures 

 vanishes distinctly earlier (p. 390). 



Literature p. 483 



