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. IV. THE DEVELOPMENT OF THE BLOOD-VESSELS OF THE CHICK[9]. Footnote 9: From the _Quarterly Journal of Microscopical Science_, Vol. XIII, 1873. With Plate 2. The development of the first blood-vessels of the yolk-sac of the chick has been investigated by a large number of observers, but with very discordant results. A good historical _résumé_ of the subject will be found in a paper of Dr Klein (liii. Band der _K. Akad. der Wissensch. Wien_), its last investigator. The subject is an important one in reference to the homologies of the blood-vascular system of the vertebrata. As I shall shew in the sequel (and on this point my observations agree with those of Dr Klein), the blood-vessels of the chick do not arise as spaces or channels between the cells of the mesoblast; on the contrary, they arise as a network formed by the united processes of mesoblast-cells, and it is through these processes, and not in the spaces between them, that the blood flows. It is, perhaps, doubtful whether a system of vessels arising in this way can be considered homologous with any vascular system which takes its origin from channels hollowed out in between the cells of the mesoblast. My own researches chiefly refer to the development of the blood-vessels in the pellucid area. I have worked but very slightly at their development in the vascular area; but, as far as my observations go, they tend to prove that the mode of their origin is the same, both for the pellucid and the vascular area. The method which I have principally pursued has been to examine the blastoderm from the under surface. It is very difficult to obtain exact notions of the mode of development of the blood-vessels by means of sections, though these come in as a valuable confirmation of the other method. For the purpose of examination I have employed (1) fresh specimens; (2) specimens treated with spirit, and then mounted in glycerine; (3) specimens treated with chloride of gold for about half a minute, and then mounted in glycerine; and (4) specimens treated with osmic acid. All these methods bring out the same appearances with varying clearness; but the successful preparations made by means of the gold chloride are the best, and bring out the appearances with the greatest distinctness.