12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



of fig. 5 to that of fig. 6, still the explanation given appears to be the 

 only one possible" (5). 



A somewhat extended series of transverse sections of an embryo 

 of about this age is represented in figures Oa-i. 



Figure 6a is a section through the head-fold ; it passes through the 

 extreme anterior end of the secondary folds (sf) that were de- 

 scribed, in surface view, above (figs. 5 and 6). The section was not 

 quite at right angles to the long axis of the embryo, so that the fold 

 on the right was cut further toward its anterior end than was the 

 fold on the left. The pushing under of the head causes a forward 

 projection of the secondary folds, so that the fold to the right ap- 

 pears as rounded mass of cells with a small cavity near its center. 

 On the left the plane of the section passes through the posterior limit 

 of the head-fold, and shows the cells of the secondary fold contin- 

 uous with the dorsal side of the ectoderm (ec). As pointed out 

 above by Clarke, the secondary folds are here some distance apart, 

 and gradually approach each other as we proceed toward the pos- 

 terior. The entoderm (en) is here flat and takes no part in the sec- 

 ondary folds. 



In figure 6b, a short distance back of the one just described, the 

 secondary folds (sf) are much larger and are closer together. On 

 the right the section passes through the extreme limit of the head- 

 fold, so that the secondary fold of that side is still a closed circle, 

 with a few scattered cells enclosed. On the left the section is pos- 

 terior to the head-fold ; on this side the secondary fold is seen as a 

 high arch of ectoderm, with a thick mass of entoderm beneath it. 



Figure 6c represents a section which passes back of the head-fold 

 on both sides. The secondary folds (sf) are seen as a pair of ecto- 

 dermal arches continuous with each other in the middle line of the 

 embryo. The ectoderm of the folds is much thickened and grad- 

 ually becomes thinner distally. On the right the entoderm shows 

 the same thickening (en) that was shown on the left side of the 

 preceding figure. This thickened appearance of the entoderm is 

 due to the fact that the section passes through the anterior limit of a 

 tall fold of that layer, which underlies the similar fold of the ecto- 

 derm that has already been described. This secondary fold of the 

 entoderm is seen on the left side of the section. It may be traced 

 through several sections, but soon flattens out posteriorly. 



Figure 6d is a short distance posterior to the preceding. The sec- 

 ondary folds are here much less pronouncedly arched and the deep 

 groove between them is reduced to a line (/). The entoderm (en) 

 is no longer markedly arched and is closely adherent, along the 

 median plane, to the ectoderm, where there is seen the thickening 



