Ill 



PRIMARY INDUCTION 



EXPERIMENTAL DATA 



383 



also the dimension and conformations of the optic vesicles, the olfactory placodes, 

 and the epiphysis vary considerably. 



As for these acrencephalic structures, the features are distinctly different from 

 those displayed by the chordomesoblast and the parts of the neural axis 

 which it induces. Of course, according to the level of the cut, we may have either 

 two normal or nearly normal equal embryos, or one normal and the other 

 defective; this defective embryo may be located either on the primary dorsal 

 side, or on the other. Moreover, no correspondence is found between the homo- 

 nymous sides; for example, one does not find a large right optic vesicle and a 

 small left one in one embryo and the reverse in the other embryo. Again, the 

 translocations performed on blastulae or very young gastrulae reveal a strong 

 tendency to form two embryos, if the section is made about half way through 

 the dorso-marginal zone. With translocations performed on more advanced 

 gastrulae, a small acrencephalic complex can be evoked on the ventral side by 

 selective transfer of the already invaginated prechordal material, but the principal 

 embryo can nevertheless be complete. 



For a long time, I have attempted to understand the results on the basis of a 

 minute pre-existing prechordal primordium, which the cutting could include to 

 a variable degree. However, no coherent interpretation could be attained with 

 this hypothesis. Today, the more dynamic representations suggested by Okada 

 and Takaya's work seems to afford the clue which I lacked. Most probably, in 



Fig. 51. An example of equatorial translocation on a Discoglossus h\astula. (a) A large Nile 

 blue mark (circles) is placed upon the grey crescent, which is quite distinct in this species; 

 the black dots indicate the level where the blastoporal lip will appear; an horizontal cut 

 has been made, and the upper part rotated 180°. (b, b^) The embryo seen at the yolk plug 

 stage, from the animal and from the vegetal pole. I. d., the infero-dorsal part, S.v. the 

 supero-ventral part (rotated), (c) The double neurula, with open folds, (d) The double 

 embryo, showing the places where the stain was re-found in the sections. The I. d. system is 

 practically normal, while the S.v.system is only feebly cephalised. Other cases have given 

 two complete heads. It is now understood that a new prechordal primordium must be 

 re-formed from chordo-mesoblastic material (from Dalcq, 1947b). 



Literature p. 483 



