i6o PATTERNS AND PROBLEMS OF DEVELOPMENT 



the median region. The margin of the area pellucida where yolk is being 

 digested also reduces rapidly (Fig. 54, A-E). At the stage of the first 

 somitic groove the border of the anterior end of the neural plate is just 

 beginning to fold dorsally and is so highly susceptible to the dye that, 

 when the rest of the embryo is appreciably stained, it is irreversibly in- 

 jured, and reduction in it is greatly retarded or does not occur at all; with 

 slight staining, however, it shows a very high rate of reduction. In Figure 

 54, F and G, this region is indicated by small crosses as injured. At this 

 stage the region of the node reduces somewhat less rapidly than the an- 

 terior region, and reduction progresses anteriorly and posteriorly from 

 it (Fig. 54, F, G). At stages of three to four somites the region of the 

 neural folds most susceptible to the dye is shghtly posterior to the anterior 

 end (Fig. 55, yl) ; as in the earlier stage, it is a region of very rapid reduc- 

 tion after slight staining. Except for the neural folds, the region of the 

 node reduces most rapidly with rate decreasing anteriorly and posteriorly 

 from it (Fig. 55, A~C). Somewhat more slowly reduction occurs at the 

 extreme anterior end of the head and progresses posteriorly, except for the 

 injured neural folds, which reduce slowly or not at all. At the stage of 

 eleven to twelve somites reduction is first evident anteriorly in the open 

 neural folds of the anterior head region, the region of the neuropore, and 

 the lateral regions, which are to form the optic vesicles, reducing most 

 rapidly in this anterior region (Fig. 55, D, E). These regions also stain 

 somewhat more deeply than other parts of the neural plate. Rate of re- 

 duction decreases posteriorly from the anterior end over most of the 

 embryo; but the primitive streak, still present in the posterior region, 

 appears as a second region of rapid reduction, with rate decreasing an- 

 teriorly and posteriorly from it, and the developing heart now becomes a 

 region of rapid reduction (Fig. 55, Z), £). Preceding torsion of the embryo, 

 the level at which torsion occurs is temporarily a region of rapid reduc- 

 tion. At the twenty-somite stage torsion is completed, and flexion of the 

 head is taking place. The regions of most rapid reduction at this stage 

 are the primitive streak, the auditory vesicles, the future gill slits, and 

 the region of the hindbrain, where flexion is greatest. The reduction gradi- 

 ent is now predominantly from the anterior end (Fig. 55, G, //, /) ; reduc- 

 tion in the primitive streak is relatively less rapid than earlier, and the 

 gradient does not extend very far anteriorly from it. The region of most 

 rapid reduction posteriorly is the future tail bud (Fig. 55, G, H). Figure 

 55, J, shows reduction in an early stage of the limb buds. Even before 



