158 PATTERNS AND PROBLEMS OF DEVELOPMENT 



in the apical half, sometimes extending farther basipetally on one side. 

 At the beginning of gastrulation the apical region still reacts strongly, the 

 reaction extends farther basipetally on the dorsal than on the ventral side, 

 and the dorsal lip of the blastopore shows in most cases an intensification 

 of the reaction. In the neurula the reaction is most intense anteriorly on 

 the floor of the neural plate and about the blastopore, less intense along 

 the whole dorsal region. The head in later stages reacts most intensely, 

 but the reaction occurs along the whole dorsal region. These data are of 

 special interest in relation to the widely accepted view that sulphydryl 

 compounds are essential factors in oxidation-reduction. It is perhaps also 

 of some significance that, except for stages just before and during early 

 cleavage, the regions of most intense reaction are practically identical with 

 those of highest susceptibility and most rapid dye reduction. 



Although the different lines of evidence are not in complete agreement 

 on certain points, they all show the presence of a definite spatial pattern 

 of physiological condition and activity. This pattern is on a molar or 

 regional scale, with gradation in condition from the apical pole basipetally 

 and from the dorsal region ventrally. A progressive change in physio- 

 logical condition and apparently, according to most of the data, a greater 

 increase in rate of metabolism of some kind occurs in the dorsal region 

 than elsewhere in the course of pregastrular development. Most of the 

 available evidence indicates that at the time of gastrulation the region 

 which becomes the dorsal lip of the blastopore, and after invagination the 

 inductor also, becomes the most intensely active region of the embryo. 

 The higher respiratory quotient of this region, indicating that carbohy- 

 drate metabolism is predominant, has already been noted. Since the re- 

 spiratory quotient of the whole embryo later approaches or becomes unity, 

 it is conceivable that the dorsal region, in consequence of its higher level 

 of activity, exercises a certain degree of dominance over other parts in 

 raising their metabolic levels; and with the rise in level, change in the 

 character of metabolism may perhaps occur. In fact, it is suggested by 

 Waddington, Needham, and Brachet (1936) that the change in metabo- 

 lism indicated by the increase in respiratory quotient may spread over 

 the embryo from the dorsal lip. The question whether the high metabolic 

 level of the dorsal region is concerned in its action in inducing develop- 

 ment of the neural plate after its invagination is also of fundamental im- 

 portance, but its consideration is postponed to a later chapter, in which 

 the problem of induction in general is taken up. 



