DEVELOPMENT OF THE AMERICAN ALLIGATOR — REESE 1 7 



the section passes through one of the medullary folds ( mf) at its 

 thickest part; and also explains the fact that the ectoblast is thinner 

 in the middle region (ec), where the section passes through the 

 medullar) groove, than it is farther toward the blastopore, where 

 the section cuts' the edge of the medullary folds. The outline- of 

 the middle and extreme posterior regions of the ectoblast are much 

 more irregular and ragged than is shown in the figure. The plane 

 of the section passes through the notochord (nt) in the posterior 

 region, btit not in the anterior end of the embryo., where a layer of 

 mesoblast (mes) is seen. The great size of the blastopore (blp) is 

 well shown, as is the beginning of the foregut (fg)- Comparison of 

 this figure with the more anterior transverse sections and with the 

 dorsal surface view of this stage will make the rather unusual condi- 

 tions comprehensible. 



Figure 8c is cut to one side of the median plane, distal to the 

 medullary folds. Being outside of the medullary folds, the ecto- 

 derm ( ec) is thinner and less dense than in figure 8& ; anteriorly it is 

 pushed down and back as the head-fold, and posteriorly it becomes 

 thin where it forms the dorsal boundary of the primitive streak (ps). 



The foregut (fg), as would be expected, is not so de*ep as in the 

 median section (Sb). The most striking feature of the section is 

 the presence of five mesoblastic somites (s). Each somite, especially 

 the second, third, and fourth, is made up of a mass of mesoblast 

 whose cells are compactly arranged peripherally, but are scattered 

 in the center, where a small myocoel may be seen. 



A series of transverse sections of the embryo shown in figures 8 

 and St/ is represented in figures 8d-j. 



Figure 8d is through the anterior end of the embryo; the posterior 

 edge of the amnion is cut only on one side (a). The medullary 

 folds (mf) are shown as two distinct masses of tissue, separated by 

 a considerable space from each other, both dorsally and ventrally ; 

 they are underlaid by the ectoderm of the head-fold, through which 

 the section passes. A mass of yolk (y) is shown at one side of the 

 section. 



Figure Se represents a section a short distance posterior to the 

 one just described, and passes through the short region where the 

 dorsal edges of the medullary folds have fused with each other. The 

 ventral side of the medullary groove ( mg) is, as in the preceding 

 section, still unclosed. An epidermal layer of ectoblast i ep) is now 

 distinct from the nervous layer (;//). 



Figure 8/ is through a region still farther toward the posterior 

 end. Here the medullary groove is again open above, and is still 



j — VI. 



