Ill PRIMARY INDUCTION EXPERIMENTAL DATA 369 



draws our attention: parts of the anterior brain and corresponding sense organs 

 were practically never induced by the prechordal endomesoblast, but were easily 

 obtained by inserting a transverse slice containing the anterior fourth of the chorda. 

 There is, clearly, a discrepancy with the expected results, for other investigations 

 have shown (see Mangold, 1958) a close relationship between deficience of the 

 prechordal plate and a reduction or even an imparity of the brain hemispheres, 

 eye, vesicles, and nasal pits. 



Fig. 40. Difference of inductive power between median and lateral parts of the archenteron. 



The chordomesoblast of the deutomerit (Fig. 39) has been divided into 2 median and 2 



lateral pieces, which were inserted in the blastocoele. From Raven and Kloos, 1945. 



Operations of the same type, but concerning younger donor neurulae and only 

 the anterior chordal region of the archenteric roof were performed by Raven and 

 Kloos (1945). A transverse strip of dorsal mesentoblastic material just behind the 

 prechordal area (Fig. 40) was divided into four pieces (two median and two 

 lateral) which were inserted separately in the blastocoele of early gastrulae. 

 The resulting structures were rather polymorphous. Notochord and muscle were 

 provided by the graft itself, while many structures were induced in the host 

 ectoblast: neural tissue, ganglion cells, cartilage (possibly from the graft), 

 pigment cells, mesenchyme, lateral line, sense organs, ear vesicle, nasal pit, 

 mouth with teeth (in two cases only), and atypical epiblast thickenings. Noto- 

 chord was only provided by the inserted median pieces. These also had the 

 privilege of inducing masses of nervous tissue, which were never a medullary 

 tube, but mostly a rather posterior brain, and only exceptionally a rudimentary 

 diencephalon (with paraphysis but no eyes). Two nasal pits were described, but 

 one was dubious and the other was related to a partial reduplication of the 

 primary forebrain. Induction of ear vesicle and lateral line organs conformed 

 to the nature of the grafts. The striking result was the difference between the 

 median and lateral pieces and concerned their capacity of difTerentiation (the 

 lateral ones produced only some recognizable myoblasts) and their inductive 

 power (less frequent and poor in structure in the lateral fragments). This was the 

 pertinent demonstration that a dorso-lateral gradient exists inside the young 

 archenteric roof, with some earlier "determination" of the presumptive noto- 



Literature p. 483 



