GENERAL FEATURES. 287 



by the shading. At the middle of the embryonic rim is to be 

 seen the rudiment of the embryo (m. g.). It consists of an 

 area of the blastoderm, circumscribed on its two sides and at 

 one end, by a slight fold, and whose other end formsjpart of 

 the edge of the blastoderm. The end of the embryo which 

 points towards the centre of the blastoderm is the head end, 

 and that which forms part of the edge of the blastoderm is 

 the tail end. To retain the nomenclature usually adopted 

 in treating of the development of the Bird, the fold at the 

 anterior end of the embryo may be called the liead fold, and 

 those at the sides the side folds. There is in Elasmobranchs 

 no tail fold, owing to the position of the embryo at the peri- 

 phery of the blastoderm, and it is by the meeting of the three 

 above-mentioned folds only, that the embryo becomes pinched 

 off from the remainder of the blastoderm. Along the median 

 line of the embryo is a shallow groove (m.g.}, the well-known 

 medullary groove of vertebrate embryology. It flattens out 

 both anteriorly and posteriorly, and is deepest in the middle 

 part of its course. 



C. 



This embryo resembles in most of its features the embryo 

 last described. It is, however, considerably larger, and the head- 

 fold and side-folds have become more pronounced structures. 

 The medullary groove is far deeper than in the earlier stage, and 

 widens out anteriorly. This anterior widening is the first indica- 

 tion of a distinction between the brain and the remainder of the 

 central nervous system, a distinction which arises long before 

 the closure of the medullary canal. 



D. 



This embryo is far larger than the one last described, but 

 the increase in length does not cause it to project beyond the 

 edge of the blastoderm, but has been due to a growth inwards 

 towards the centre of the blastoderm. The head is now indicated 

 by an anterior enlargement, and the embryo also widens out 

 posteriorly. The posterior widening (t. s.} is formed by a pair of 

 rounded prominences, one on each side of the middle line. These 

 are very conspicuous organs during the earlier stages of develop- 

 ment, and consist of two large aggregations of mesoblast cells. 



