4SO PATTERNS AND PROBLEMS OF DEVELOPMENT 



that they apparently occur first and are greatest in those regions of the 

 embryo which show the highest differential susceptibility, the most rapid 

 dye reduction, and perhaps the highest respiration (pp. 151-58), and the 

 stretching is greatest in the longitudinal direction. These relations sug- 

 gest that they are related to the gradient pattern of the stages concerned. 

 It appears difficult to account for stretching in a particular direction unless 

 there is a differential of some kind in this direction, affecting individual 

 cells. The isolated and transplanted pieces, or even individual cells, rep- 

 resent a part of this differential and behave accordingly. As Vogt and 

 Spemann have pointed out, the movements constitute an orderly, defi- 

 nite series of events which determine a definite result. If the cells were all 

 alike and there were no regional directive factor, this would be impossible. 

 Such evidence as is available indicates that the cells initiating invagina- 

 tion in the intact embryo represent the highest levels of the gradient 

 system of the dorsal lip region. In the absence of this region in trans- 

 planted pieces the highest gradient-level present apparently initiates in- 

 vagination. Increase in surface area of cells occurs in early embryonic 

 development of many other forms and apparently begins in the high 

 regions of the gradient pattern present. For example, the ectoderm of 

 the echinoderm blastula and gastrula undergoes increase in surface area, 

 beginning apically and progressing basipetally, and the developmental 

 activation of the entoderm is followed by increase in its surface area. The 

 relations of these changes to gradient pattern are evident; and when the 

 gradient pattern is experimentally altered or obliterated, they are also 

 altered. 



In the amphibian the regional differences in these cell activities appear 

 to be quantitative rather than limited to particular regions, but they 

 may be regarded as representing a certain stage of development from the 

 primary gradient pattern, this stage being attained first and in greatest 

 degree by cells of the higher gradient-levels. Whether, or to what ex- 

 tent, the cells concerned have become specifically different from others 

 which attain the condition later or not at all remains to be determined. 

 The autonomous invagination of pieces of the blastopore lip suggests a 

 "determination" of some sort. The factors determining invagination are 

 unknown, but, in general, gastrulation appears to represent a certain 

 stage in the progressive complication of the earlier gradient pattern. 

 This seems particularly clear in echinoderm development. 



The regional map of the teleost blastoderm, as determined by Pasteels 

 (1936a), is shown in Figure 152, A; and the directions of cell migrations 



