l62 



PATTERNS AND PROBLEMS OF DEVELOPMENT 



The reduction gradient in three stages of the developing heart is shown 

 in Figure 56, A-C. The posterior region, where the omphalomesenteric 

 veins fuse, shows the highest rate with progressive decrease anteriorly, 

 but with higher rate on the right side (left in the figures). 



Studies of differential susceptibility to cyanide, ammonia, and sodium 

 hydroxide by Hyman (1927a), to ultra-violet radiation by Hinrichs (1927), 



B 



c 



Fig. 56, .4-C.— Differential dye reduction in three developmental stages of chick heart 

 (fromRulon, 1935). 



and to hydrocyanic acid by Buchanan (1926^) are in essential agreement, 

 not only with each other but with the results of differential dye reduction, 

 so far as the same stages are concerned. In primitive-streak and head- 

 process stages a simple anteroposterior gradient appears. From early neu- 

 ral fold stages on, the anterior region and the region of the node are, in 

 general, most susceptible; but other regions of high susceptibility appear, 

 some temporarily. In early somite stages the region of closure of the 



