24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



region represented in figure gm. As was noted in the preceding 

 stage, the medullary folds are quite continuous with the folds of the 

 primitive streak, and the medullary groove with the primitive 

 groove ; so that, unless we take the dorsal opening of the neurenteric 

 canal as the point of separation, there is no line of division between 

 these structures. The entoderm (en) and the lateral regions of the 

 ectoderm (ec) and mesoderm (mes) in figure gm are about as they 

 were in figure g\, but in the middle line is seen a compact mass of 

 cells forming the primitive streak (ps), with the shallow primitive 

 groove (pg) on the dorsal side. The cells on each side of the prim- 

 itive groove and for a short distance below it are compact, as is 

 shown in the figure, but as we pass ventrally and laterally they be- 

 come looser and more angular to form the lateral sheets of meso- 

 blast ( mes), very much as is the case in the chick and other forms. 

 For a few sections posterior to the one shown in figure gm the prim- 

 itive streak may be seen, then it disappears, and only the ectoderm 

 and entoderm remain as thin sheets of tissue above the yolk. 



Stage VII 



Figures 10 and 10a (Plates X, XI) 



Although of practically the same size as the preceding, this stage 

 has advanced sufficiently in development to warrant a description. 



The medullary folds are apparently still slightly open for the 

 greater part of their length, though they are evidently fused together 

 in the head region, except at the extreme end. Transverse sections, 

 however, of figure 12, in which the medullary folds, from the dorsal 

 aspect, seemed open (mg) as in figure 10, have shown that these 

 folds are fused throughout their length. 



The first cerebral vesicle (v') is clearly indicated as an enlarge- 

 ment of the anterior end of the nervous system, and a slight enlarge- 

 ment (v") posterior to the first probably represents the second 

 cerebral vesicle. 



There are now eight pairs of somites (s). 



The head-fold (//) now shows in both dorsal and ventral views, 

 appearing in the former, when viewed by transmitted light, as a 

 lighter, circular area on either side of the body, just posterior to the 

 hinder edge of the amnion. 



The head-fold of the amnion (a) has extended about twice as far 

 backward as it did in the preceding stage. 



Owing to the opacity caused by the medullary folds being in con- 

 tact along the middle line, the notochord is no longer visible in sur- 

 face views. 



