8 CHIM^ROID FISHES AND THEIR DEVELOPMENT. 



Into this region extends a delicate layer of mesoblast (W); and here and there 

 groupings of cells of this layer, as at v, suggest the formation of blood vessels. 

 Under the gigantic cell, one notes that the cells of the mesoblast layer are of 

 remarkable size. 



Another section, given in fig. 70 B, pictures details of a section similar to fig. 

 69 G. This illustrates particularly a subgerminal zone containing large yolk nuclei ; 

 of these some are situated close to the surface of the subgerminal wall, and one (W) 

 has passed into the entoderm. This obviously cannot be confused with the adja- 

 cent entoderm cells, if only on account of its greater size. In this section a special 

 area of formative yolk is shown underlying the periphery of the blastoderm. Under 

 the embryo itself the formative yolk attains the surface notably at the sides of the 

 embryonic body, and it is from this region that the cells appear to be passed into 

 the embryo. Less activity is probably present in the ventral median line, on 

 account of the quantity of coarse yolk which is here present. 



LATER GASTRULA. EMBRYO WITH OPEN MEDULLARY FOLDS. 



This stage, figured in surface view, plate v, fig. 36, and in detail, plate vi, fig. 40, 

 may be compared with Balfour's stage D in elasmobranch. In spite of the conspicu- 

 ous growth of the embryo, the blastoderm, it may be noted, remains remarkably 

 small in size. In this stage the blastoderm of Chimaera shows a well-marked 

 central area, which on closer examination is found to be made up of spongy mesh- 

 work ; there is also a somewhat thickened rim, and a marginal zone, the latter 

 shown in sections to be formed of peristomial mesoblast. Beyond the limits of the 

 blastoderm the surface of the yolk showed faintly diverging lines which suggested 

 cleavage planes. (Cf. plate vi, fig. 39 A.) The embryo itself, when viewed as a 

 transparent object, plate vi, fig. 40, shows shark-like medullary folds, more delicate, 

 however, and narrower in proportions. The tail folds are less conspicuous ; the 

 mesoblast concentrating in this region shows on each side a dark area, the rela- 

 tions of which are referred to later. 



DETAILS OF STAGE D. 



Transverse sections of the embryo and the neighboring blastoderm in this 

 stage are pictured in fig. 71, A-i. Thus beginning with a section through the tail 

 folds, we see in B, ectoderm and entoderm continuous in the chordal region. In 

 this section the mesoderm merges with the entoderm not at the sides of the chordal 

 region, but near the margin of the blastoderm, thus suggesting the theoretical condi- 

 tion in the origin of the mesoblast advocated by Graham Kerr. In section c (at the 

 left side, the plane being slightly oblique) the side of the blastoderm is coming into 

 functional connection with the yolk; the notochord is here being folded off from 

 the entoderm ; the latter is now a thick, flattened layer, its outer half lying apposed 

 to the yolk wall. In D the section shows the beginning of the neural folds ; below 

 them is a well formed layer of mesoblast, also the dorsal wall of the gut; the gut 

 lumen appears at g ; at its side the dorsal wall of the gut shows a wide contact with 



