DEVELOPMENT OF ELASMOBRANCH FISHES. 73 



line. These are very conspicuous organs during the earlier 

 stages of development, and consist of two large aggi'egations of 

 mesoblast cells. In accordance with the nomenclature adopted 

 in my preliminary paper^ 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, al- 

 though 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 fre&h 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 de- 

 ceptive, since in sections no groove, or at most a very slight 

 one, is perceptible. 



1 Quart. Journ. Jlicr. Science, Oct. 1874. 



