Figure 6_c_ represents a section which passes back of the head-fold on both sides. The secondary folds (_sf_) are seen as a pair of ectodermal arches continuous with each other in the middle line of the embryo. The ectoderm of the folds is much thickened and gradually 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 ectoderm 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 6_d_ is a short distance posterior to the preceding. The secondary folds are here much less pronouncedly arched and the deep groove between them is reduced to a line (_l_). 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 (_th_) that has been mentioned by Clarke (see above). Springing from the entoderm on each side of this thickening is a small mass of mesoderm (_mes_). The section immediately posterior to the one just described is represented in Figure 6_e_. The line (_l_) which separated the two secondary folds in the preceding section is no longer present, so that the ectoderm (_ec_) is continuous from side to side, with only a shallow depression (_mg_), which may be considered as the extreme anterior end of the medullary groove. The median thickening (_th_) is cut near its posterior limit and still shows a close fusion of the germ layers. There is no line of demarcation between the gradually flattening secondary folds of the anterior end of the embryo and the just forming medullary folds of the posterior end, so that it is impossible to say whether the thickening of ectoblast in this figure should be called secondary folds or medullary folds. As a matter of fact, the secondary folds become, of course, the anterior ends of the medullary folds. The mesoblast (_mes_) is here of considerable extent, and its entodermal origin is beyond doubt, though not well shown in the figure. Figure 6_f_ is about one sixth of the length of the embryo posterior to the preceding. The medullary thickening of the ectoderm (_ec_) is still marked and the shallow medullary groove (_mg_) is fairly distinct. The entoderm (_en_) is medially continuous with both mesoderm (_mes_) and notochord (_nt_), though these two tissues are otherwise distinct from each other.