The invaginated cells forming the dorsal wall of the mesenteron soon become divided into a pigmented hypoblastic epithelium adjoining the lumen of the mesenteron (fig. 51, _En_) and a mesoblastic layer (_Sgp_), between the hypoblast and the epiblast. The mesoblast is divided into two plates, between which is placed the notochord[36] (_Ch_). [36] Salensky believes that the notochord is derived from the mesoblast. I could not satisfy myself on this point. With the completion of the medullary plate and the germinal layers, the first embryonic period may be considered to come to a close. The second period ends with the hatching of the embryo. During it the rudiments of the greater number of organs make their appearance. The general form of the embryo during this period is shewn in figs. 50 B and 52 A and B. One of the first changes to take place is the conversion of the medullary plate into the medullary canal. This, as shewn in fig. 51, is effected in the usual vertebrate fashion, by the establishment of a medullary groove which is then converted into a closed canal by the folding over of the sides. The uncovered patch of yolk in the blastoporic area soon becomes closed over; and on the formation of the medullary canal the usual neurenteric canal becomes established. [FIG. 51. TRANSVERSE SECTION THROUGH THE ANTERIOR PART OF AN ACIPENSER EMBRYO. (After Salensky.) _Rf._ medullary groove; _Mp._ medullary plate; _Wg._ segmental duct; _Ch._ notochord; _En._ hypoblast; _Sgp._ mesoblastic somite; _Sp._ parietal part of mesoblastic plate.] The further changes which take place are in the main similar to those in other Ichthyopsida, but in some ways the appearance of the embryo is, as may be gathered from fig. 52, rather strange. This is mainly due to the fact that the embryo does not become folded off from the yolk in the manner usual in Vertebrates; and as will be shewn in the sequel, the relation of the yolk to the embryo is unlike that in any other known Vertebrate. The appearance of the embryo is something like that of an ordinary embryo slit open along the ventral side and then flattened out. Organs which properly belong to the ventral side appear on the lateral parts of the dorsal surface. Owing to the great forward extension of the yolk the heart (fig. 52 B) appears to be placed directly in front of the head. Even before the formation of the medullary canal the cephalic portion of the nervous system becomes marked out. This part, after the closure of the medullary groove, becomes divided into two (fig. 50 B), and then three lobes--the fore-, the mid-, and the hind-brain (fig. 52, A and B). From the lateral parts of the at first undivided fore-brain the optic vesicles (fig. 52 B, _Op_) soon sprout out; and in the hind-brain a dilatation to form the fourth ventricle appears in the usual fashion. [FIG. 52. EMBRYOS OF ACIPENSER BELONGING TO TWO STAGES VIEWED FROM THE DORSAL SURFACE. (After Salensky.)