Ill PRIMARY INDUCTION EXPERIMENTAL DATA 387 



The kinematic behaviour of the graft confirmed what has been seen in Discoglossus gastru- 

 lation. The original antipolar material of the transplant migrates directly through the 

 depth of the ventral zone toward the pole, without tumbling over a blastoporal lip, 

 and is followed by the previously polar part. An atypical blastoporal lip nevertheless 

 appears in the host mesoblast or ectoblast, and its movement helps to cover the implant 

 (Fig. 54,a-f). By this kinematic regulation (p. 379) both parts of the graft nevertheless file 

 in a normal sequence, but the migration is slowed down and the obtained embryonic axis 

 remains slender and often atypical. Especially the forebrain is poorly developed (Fig. 

 54,g, j) and in no case has the prechordal plate expressed its full inductive capacity. 



One general observation in these experiments, as in the above mentioned 

 translocations, has been the greater activity of the medio-dorsal material in 

 contrast to the more lateral one. Grafts containing presumptive notochord 

 material are tmdoubtedly superior in all aspects of activity to purely somitic ones 

 (compare Lallier's and Minganti's results). This remark, which will also be 

 supported, for bird embryos, by Mulherkar's results (p. 410), refutes the criticism 

 of Holtfreter an Hamburger who write (1955, their p. 243) . . ."it is furthermore 

 arbitrary to assume that the prospective notochord possesses a higher concen- 

 tration of this substance (organisine) hence a higher 'morphogenetic potential' 

 than the prospective somites." (for the concept of organizin, see p. 478). 



Before passing to operations of a quite different type, a reference must be made to a paper of 

 Waechter (1953). It is an extensive description of experiments largely initiated by Mangold 

 (1936) and consisting of heteroplastic insertions (in theblastocoele) of fragments cut from the 

 central part of the neural plate. The intrinsic development of the implants varied according 

 to their origin, as did also, more or less, the inductions they evoked. From both points of 

 view, a medio-lateral gradient was manifest. 



We now come to a difTerent, quite original trend of experiments. They were 

 conceived by the Dutch embryologist Nieuwkoop and performed, from 1947 on, 

 by himself and a series of guests at the Hubrecht Laboratory (Utrecht). 



Three diflferent types of experiments have been studied by this team of workers : 

 a first group on whole embryos, a second group with normal explants reared in 

 ectoblast sandwiches, and a third with killed inductors, also inserted in sand- 

 wiches. According to our plan, we shall be forced to dissociate these three groups, 

 a procedure which I think will be beneficial. 



Nieuwkoop's principal operations (1952) have consisted of implanting a folded 

 piece of still indifferent ectoblast in the neural field of various Urodeles (Fig. 55). 

 Xenoplastic combinations were frequently used. When performed on young 

 gastrulae, this addition does interfere with gastrular morphochoresis ; the superior 

 part of the chordomesoblastic territory becomes neurectoblastic, while the upper 

 part of the neural territory forms only epiblast (Fig. 56). The essential cause is 

 that the immigration of the dorso-marginal material is reduced and the archen- 

 teron shortened. The graft does not persist for a long time in this flat folded state. 

 Its base swells and its attachment expands or contracts depending on the case. 

 The ultimate transformation depends upon the location of the fold and the age 

 of the host. 



It is considered certain that all implants made outside the netiral field yield only 

 atypical ectoblast, eventually with one or more balancers at their root (Fig. 57, a). 



Literature p. 483 



