All this can easily be seen with a simple microscope, but the sections taken from the specimen figured also fully bear out the interpretations given above, and at the same time shew that the notochord has at this age begun to appear. The sections marked 13-17 pass respectively through the lines with corresponding numbers in fig. 18. Section 1 (fig. 13) passes through the middle of the medullary canal.
In it the following points are to be noted. (1) That the epiblast becomes very much thinner where it lines the medullary canal (_mc_), a feature never found in the epiblast lining the primitive groove. (2) That the mesoblast is very much thickened to form the medullary folds at _A_, _A_, while there is no adherence between it and the epiblast, below the primitive groove. (3) The notochord (_ch_) has begun to be formed, though its separation from the rest of the mesoblast is not as yet very distinct[8].
Footnote 8: In the figure the notochord has been made too distinct.
In fig. 14 the medullary groove has become wider and the medullary folds broader, the notochord has also become more expanded: the other features are the same as in section 1. In the third section (fig. 15) the notochord is still more expanded; the bottom of the now much expanded medullary groove has become raised to form the ridge which separates the medullary from the primitive groove. The medullary folds are also flatter and broader than in the previous section. Section 4 (fig. 16) passes through the anterior end of the primitive groove. Here the notochord is no longer visible, and the adherence between the mesoblast and epiblast below the primitive groove comes out in marked contrast with the entire separation of the two layers in the previous sections.
The medullary folds (_A_) are still visible outside the raised edges of the primitive groove, and are as distinctly as possible separate and independent formations, having no connection with the folds of the primitive groove. In the last section (fig. 17), which is taken some way behind section 4, no trace of the medullary folds is any longer to be seen, and the primitive groove has become deeper. This series of sections, taken in conjunction with the specimen figured in fig. 18, must remove all possible doubt as to the total and entire independence of the primitive and medullary grooves. They arise in different parts of the blastoderm; the one reaches its maximum growth before the other has commenced to be formed; and finally, they are distinguished by almost every possible feature by which two such grooves could be distinguished.