KINETIC AND CAUSATIVE ASPECTS 



345 



that a given threshold will be surpassed earlier in the region of maximal interaction, and 

 that the process will be gradually extended in a preferential direction to nearby territories. 

 It would also explain why the lines of disjunction, i.e. the limits of the presumptive areas, 

 display on the blastula or nearly gastrula such a geometrical design. 



This proposal has not been generally accepted since it has been considered too speculative 

 (Rotmann, 1941). This criticism is no longer completely justified, for each set of factors 

 has been elaborately analyzed and explored as far as now possible by the authors of the 

 theory. Their opponents were forced to propose an alternative explanation of the experi- 

 mental results, so they have claimed that the organizer is in some way present in the 

 dorsal marginal zone, and that the forced inversions only draw this active plasm into 

 abnormal locations (Lehmann, 1942b). This conception is hardly compatible with a 

 sound epigenetic interpretation. In support of the original theory, an appropriate experi- 

 ment of compulsory inversion was performed whose results could not be explained by the 

 transfer of previously formed material (Pasteels, 1946). It has also been suggested that 

 the patch of yolk cells would only provide a resistant basis for the folding and movement 

 of the adjacent sheet of brown cells (see Holtfreter and Hamburger, 1955, their p. 234). This 

 mechanical explanation would not account for the striking effect of the relation with the 

 grey crescent. 



A more real difficulty lies in the fact that there is at least one case in which a blastoporal 

 lip arises de novo and for which an interaction between yolk and cortex cannot be advocated. 



VEG.^-%j., 



Fig. 32. Gastrulation in previously inverted frog eggs. After the manoeuvre shown in 

 Fig. 30 (c-e) the location of the grey crescent was marked on the slides by glass pencil 

 (indicated here by an asterisk). VEG and AN refer to the connotations before inversion (as 

 in Fig. 30, c). The pigmented zones are dotted and the areas where yolk is located are white. 

 (a) The yolk has flowed toward the grey crescent ; the blastoporal lip appears in the middle 

 of the yolk mass, (b) The yolk forms a girdle, widest on the vegetative side, with a denser 

 central area; the lip appears on the side of this mass toward the grey crescent, (c) The 

 yolk has partly adhered to the animal region and partly attained the vegetative part, each 

 time with a central mass. The lip appears simultaneously near both masses, each time on 

 the side turned toward the grey crescent. The rule is thus that morphochoresis begins in 

 relation with the grey crescent, therefrom in relation to the cortical field, centred on the 

 grey crescent and not affected by inversion. From Pasteels, 1938. 



Literature p. 483 



