ON THE DEVELOPMENT OF THE ELASMOBRANCH FISHES. ^35 



first appear to mark out the portion of the germ from which 

 the embryo becomes formed; this period would be in the 

 Elasmobranchii, as in the Osseous fish, at the termination of 

 segmentation^ when the want of symmetry between the 

 embryonic end of the germ and the opposite end first ap- 

 pears. 



I described in the last stage the formation of the ''em- 

 bryonic rim." It is in the middle point of this, where it 

 projects most, that the development of the embryo takes 

 place. There appear two parallel folds extending from the 

 edge of the blastoderm towards the centre, and cut off at 

 their central end by another transverse fold. These three 

 folds raise up, between them, a flat broadish ridge, " the 

 embryo^' (PI. XIV, fig. 5). The head end of the embryo 

 is the end nearest the centre of the blastoderm, the tail end 

 being the one formed by its (the blastoderm's) edge. 



Almost from its first appearance this ridge acquires a 

 shallow groove — the medullary groove (PL XIV, fig. 5, w, g) 

 — along its middle line, where the epiblast and hypoblast are 

 in absolute contact {vide fig. Qa,l a,l b, &c.), and where the 

 mesoblast (which is already formed by this stage) is totally 

 absent. This groove ends abruptly a little before the front 

 end of the embryo, and is deepest in the middle and wide 

 and shallow behind. 



On each side of it is a plate of mesoblast equivalent to the 

 combined vertebral and lateral plates of the Chick. These, 

 though they cannot be considered as entirely the cause of the 

 medullary groove, may perhaps help to make it deeper. In 

 the parts of the germ outside the embryo the mesoblast is 

 again totally absent, or, more correctly, we might say that 

 outside the embryo the lowei^ layer cells do not become differ- 

 entiated into hypoblast and mesoblast, and remain continuous 

 only with the lower of the two layers into which the lower 

 layer cells become differentiated in the body of embryo. This 

 state of things is not really very different from what we find 

 in the Chick. Here outside the embryo (i. e. in the opaque 

 area) there is a layer of cells in which no differentiation 

 into hypoblast and mesoblast takes place, but the layer 

 remains continuous rather with the mesoblast than the 

 hypoblast. 



There is one peculiarity in the formation of the mesoblast 

 which I wish to call attention to, i. e. its formation as two 

 lateral masses, one on each side of the middle line, but not 

 continuous across this line [vide figs. 6 a and 6 6, and 7 a and 

 7 b) . Whether this remarkable condition is the most primi- 

 tive, ^. e. whether, when in the stage before this the mesoblast 



