28'8 DEVELOPMENT OF ELASMOBRANCH FISHES. 



In accordance with the nomenclature adopted in my preliminary 

 paper 1 , they may be called 'tail-swellings'. Between the cephalic 

 enlargements and the tail-swellings is situated the rudimentary 

 trunk of the embryo. It is more completely pinched off from 

 the blastoderm than in the last described embryo. The 

 medullary groove is of a fairly uniform size throughout the 

 trunk of the embryo, but flattens out and vanishes completely 

 in the region of the head. The blastoderm in Pristiurus and 

 Scyllium grows very rapidly, and has by this stage attained 

 a very considerable size ; but in Torpedo its growth is very 

 slow. 



E and F. 



These two embryos may be considered together, for, although 

 they differ in appearance, yet they are of an almost identical 

 age ; and the differences between the two are purely external. 

 E appears to be a little abnormal in not having the cephalic 

 region so distinctly marked off from the trunk as is usual. The 

 head is proportionally larger than in the last stage, and the tail- 

 swellings remain as conspicuous as before. The folding off from 

 the blastoderm has progressed rapidly, and the head and tail are 

 quite separated from it. The medullary groove has become 

 closed posteriorly in both embryos, but the closing has extended 

 further forwards in F than in E. In F the medullary folds have 

 not only united posteriorly, but have very nearly effected a fresh 

 junction in the region of the neck. At this point a second 

 junction of the two medullary folds is in fact actually effected 

 before the posterior closing has extended forwards so far. The 

 later junction in the region of the neck corresponds in position 

 with the point, where in the Bird the medullary folds first unite. 

 No trace of a medullary groove is to be met with in the head, 

 which simply consists of a wide flattened plate. Between the 

 two tail-swellings surface views present the appearance of a 

 groove, but this appearance is deceptive, since in sections no 

 groove, or at most a very slight one, is perceptible. 



G. 



During the preceding stages growth in the embryo is very 

 slow, and considerable intervals of time elapse before any 



1 Quart. Jotirn. Mur. Sciencf, Oct. 1X74. [This Edition, No. V.] 



