REGULATION IN ANURAN EMBRYOS 185 



uation? The embryo finds itself unable to elongate directly, and 

 yet continuous at its posterior end with two stripes of indifferent 

 tissue. These are accordingly, under the general influence of the 

 regulatory tendency, organized through the action of the struc- 

 tures in front, precisely as the growing tail bud is organized in 

 the normal embryo. The difference is that in the spina-bifida 

 embryo the entire stripe is there from the start, while the cord 

 of cells, to which the backwardly growing tail bud gives rise, 

 is formed and metamorphosed into its several derivatives 

 gradually. 



There are still other ways in which we may conceive of spina- 

 bifida embryos as being formed. Kopsch ('96, '99, '04) and H. 

 V. Wilson ('00), for instance, have held that it is not necessary 

 to regard them as due to organization in situ of the blastopore 

 lips, but that they may be produced as the result of a progressive 

 splitting of the axial body. This explanation may well apply to 

 some cases. 



It is plain that spina-bifida embryos of themselves will not tell 

 us whether they have been built up by normal or abnormal proc- 

 esses. If the blastopore lip organizes, we may claim with one 

 school that it is a normal process, with the other that the lip has 

 been activated perhaps in the manner described above. If it 

 does not organize, as may happen, we may adduce this as proof 

 that there is no normal impulse in it toward organization, or 

 with the other school that the normal impulse has been inhib- 

 ited. In the midst of this debating one does not forget, however, 

 that the proximate, so-called practical, thing to do is to learn how 

 to control the differentiation of the lip, to call out in it, or pre- 

 vent perhaps, the formation of a neural tube, notochord, and 

 somites. 



ready differentiated cells" into the indifferent mass, falls in Roux's category of 

 'dependent differentiation' ('abhangige Differenzirung'). The directive force 

 Roux designates 'an assimilative and differentiative action,' (loc. cit., p. 509). 

 To it is applicable the term 'morphological assimilation' (Roux, '12, p. 28), 

 which might be used, pending the coinage of an appropriate Greek word. Other 

 embryologists had already postulated the existence of such an influence in nor- 

 mal development, but Roux, I believe, for the first time gave precision to the 

 idea. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 30, NO. 2 



