Ill 



PRIMARY INDUCTION — ■ EXPERIMENTAL DATA 



381 



A more extensive operation was then endeavoured, to which the Discoglossus 

 eggs resisted pretty well. It consisted of a thorough modification of the topography 

 in the whole marginal zone, by the technique called {sub-) equatorial translocation. 

 The blastula or young gastrula is cut horizontally at the level of the blastocoele 

 floor, the upper part turned through 180" and replaced on the vegetal half 

 Spemann (1918) was the first to use this technique on newt eggs, but owing to the 

 low position of their dorso-marginal zone and possibly to some other reason, he did 

 not obtain double embryos. This negative result was, however, very important, 

 for it suggested to Spemann that an extrinsic factor was responsible for obtaining 

 the neural plate, and led him as a next step to the crucial experiment of trans- 

 planting the blastoporal lip. It occurred to me that frog eggs would not react in 

 the same way. As long as this technique divided the dorso-marginal zone, it 

 resulted in double embryos, which exhibited a correspondance worth of a more 

 extensive study, which could be best done on Discoglossus eggs, whose consistency 

 permitted this rather drastic operation without appreciable loss of material 

 (Fig. 51). This fast-developing material shows most convincingly that as early 

 as the late blastula stage the dorso-marginal zone is no more a continuum (with 

 graded properties), but that the notochordal, somitic, and pronephritic territories 

 have acquired some properties of their own. These tendencies can express them- 

 selves in small parts of the presumptive area when isolated from their normal 

 connections. Such intrinsic and rather stable qualities explain the striking 

 correspondance between left and right parts, respectively, of the two axial systems 

 obtained. This interpretation can even be extended to the production of the two 

 groups of cells responsible for the primary induction of the otocystes, located 

 in the parachordal head mesoblast (Dalcq, 1933). Moreover, in the majority 

 of cases, both embryos possess a forebrain. The degree of its perfection and 



br.o. 



Fig. 49. Computation of the results produced by an in situ i8o°-rotation of the central part 

 of the dorso-marginal zone, in an early Discoglossus gastrula. (A) Map of presumptive areas 

 with the heavy line indicating the extent of the cut. Note that the cut has been slightly 

 asymmetrical, shifted from 10 to 20° to the right; bl., line which will be followed by the 

 blastopore, now limited to its youngest part, y.b.; br. o., branchial openings; ep., epiblast; 

 1., limit between endoblast (below) and chordomesoblast (above); n.c, notochord; n.e., 

 neurectoblast; p.p., prechordal plate; som., somites ;x,y, z, three particular areas indicated 

 to draw attention to their change in position (see legend of Fig. 50, c) . (B) the pattern of 

 presumptive areas after the rotation. From Dalcq, 1941a. 



Literature p. 483 



