]8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



open below. A well-marked space is seen between the epidermal 

 (ep) and nervous (nl) layers of the ectoderm, but no mesoblast is 

 yet to be seen. 



Figure 8g passes through the middle part of the head-fold, and 

 shows that the medullary folds in this region are fused below, but 

 are widely separated above, where their margins are markedly bent 

 away from the mid-line. Between the epidermal and nervous layers 

 of the ectoderm a considerable mass of mesoderm cells (mes) is 

 seen. The curious appearance of the preceding four figures, as well 

 as the first three figures of the next stage, was at first quite puzzling, 

 until a model of the embryo was made from a series of sections. It 

 was then plain, as would have been the case before, except for the 

 unusual depth dorso-vcntrally of the head of the embryo, why the 

 medullar}- canal should at the extreme anterior end be open both 

 dorsally and ventrally, while a few sections caudad it is open only 

 ventrally, and still farther toward the tail it is again open both above 

 and below. These conditions are produced by the bending under of 

 the anterior region of the medullary folds, probably by the formation 

 of the head-fold. It is apparently a process distinct from the ordi- 

 nary cranial flexure, which occurs later. The fusion of the medul- 

 lary folds to form a canal begins, as has been already mentioned, 

 near the anterior end, whence it extends both forward and backward. 

 Hence, if the anterior ends of the medullary folds be bent downward 

 and backward, their unfused dorsal edges will come to face ventrally 

 instead of dorsally, and sections through the anterior part of this 

 bent-under region will show the medullary canal open both above 

 and below, as in figure S</. while sections farther caudad pass 

 through the short region where the folds are fused, and we have the 

 appearance represented in figure 8c. In figure 8/ is shown a section 

 passing posterior to the short, fused region of the folds, and we 

 again have the medullary canal open both above and below. Figure 

 8g represents a section through the tip of the bent-under region of 

 the medullary folds, which are here fused below and open above. 



Figure 8// passes through the posterior part of the head-fold, be- 

 tween the limits of the fold of the ectoderm and that of the ento- 

 derm. The medullary groove (mg) is here very wide and compar- 

 atively shallow ; its walls are continued laterally as the gradually 

 thinning ectoderm (ec) . The enteron (cnt) is completely enclosed, 

 and forms a large, somewhat compressed, thick-walled cavity. Be- 

 tween the dorsal wall of the enteron and the lower side of the medul- 

 lary canal lies the notochord I nt), a small, cylindrical rod of closely 

 packed cells derived, in this region at least, from the entoderm. In 

 the posterior region of the embryo it is not possible to determine 



