Ill PRIMARY INDUCTION EXPERIMENTAL DATA 4O9 



shows that cell division occurs practically at the same rate in both cases. The difference 

 between a fresh saline medium and a conditioned one does not depend upon the presence 

 of growth-promoting, but of differentiating agents. 



Further analysis has shown that pn change or other precytolytic factors could be 

 excluded, since the reaction differed among species, being the most active in Ambystoma, 

 less in Triturits, T. rividaris being preferable to torosus. In the case of T. rivularis, 24-h. time 

 in the conditioned medium is sufficient to obtain induction. The relative efficiency of 

 the inductors has been tentatively expressed as follows : 



dorsal lip ^ posterior region of medullary plate > notochord ^ medullary plate and 

 fold > neural fold > somites > endoblast. The nature of the change introduced in the 

 saline medium by the cultivation of the explant will naturally be considered when we 

 examine the intimate nature of induction (p. 476). 



Fig. 70. Effect of conditioned medium. See text. From Niu, 1956. 



Some operations of explantation and grafting have been performed on bird 

 blastoderms. In this material, the typical culture in vitro of isolated fragments is 

 still less favorable than in amphibians. Preference had to be given to the chorio- 

 allantoic grafts which have been extensively studied (Hunt, Rawles, Rudnick, 

 literature in Rudnick, 1955). The same general process of paragenesis is observed, 

 with a maximum of complexity for pieces originating from the mediodorsal area, 

 if they are of a sufficient size. 



The nutritional requirements of bird blastoderms have been established thanks 

 to the extensive work of Spratt (1952). Working on the duck egg, Lutz and Oster- 

 tag (1955) have observed that non-incubated blastoderms are unfavorable for ex- 

 plantation but that when placed in contact with a piece of endoblast (taken from 

 primitive streak stages) local morphochoresis is induced in them with all its conse- 

 qtiences. This is a confirmation of the older data of Waddington concerning the 

 inductive activity of the endophyll. 



The difficulty of cultivation has now been overcome by the very practical 

 method of New (1955) in which the blastoderm is simply deposed on a stretched 

 piece of the vitelline membrane and covered with albumen. Using this technique, 

 Mulherkar (1958) has methodically explored the inductive capacity of minute 

 regions surrounding the anterior node at the stage where the definite primitive 

 streak becomes perceptible. The results, represented in Fig. 71, show a 

 maximum of induction capacity in the node itself, and a decrease not only 

 posteriorly and anteriorly, but also laterally. With regard to the laterality, there 

 is a remarkable asymmetry in favor of the left side. 



Literature p. 483 



