The second (fig. 14) section passes through the medullary groove where it has become wider. Medullary folds, A, A; notochord, ch.
In the third section (fig. 15) the notochord (ch) is broader, and the epiblast is raised in the centre, while the medullary folds are seen far apart at A.
In section fig. 16 the medullary folds (A) are still to be seen enclosing the anterior end of the primitive groove (pr). Where the primitive groove appears there is a fusion of the epiblast and mesoblast, and no appearance of the notochord.
In the last section, fig. 17, no trace is to be seen of the medullary folds.
Figs. 18 and 19 are magnified views of two hardened blastoderms. Fig. 18 is twenty-three hours old; fig. 19 twenty-five hours. They both shew how the medullary canal arises entirely independently of the primitive groove and in front of it, and also how the primitive groove gets pushed backwards by the growth of the medullary groove. pv, Protovertebræ; other references as above. Fig. 18 is the blastoderm from which sections figs. 13-17 were cut.
[7] From the Quarterly Journal of Microscopical Science, Vol. XIII, 1873.
[8] In the figure the notochord has been made too distinct.
IV. The Development of the Blood-vessels of the Chick[9].
With Plate 2.