74 DEVELOPMENT OF THE ELASMOBRANCH FISHES. 



I described in the last stage the appearance of the " embry- 

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

 most, that the formation 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, be- 

 tween them, a flat broadish ridge, "the embryo" (PL 3, 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 (PI. 3, fig. 5, m g) 

 along its middle line, where the epiblast and hypoblast are 

 in absolute contact (vide fig, 6 a, 7 a, 7 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 lower layer cells do not become differ- 

 entiated into hypoblast and mesoblast, and remain continu- 

 ous 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 dif- 

 ferentiation into hypoblast and mesoblast takes place, but the 

 layer remains continuous rather with the hypoblast than the 

 mesoblast. 



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 b, and 7 a and 

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



