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 indication 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 development, and consist of two large aggregations of mesoblast cells. In accordance with the nomenclature adopted in my preliminary paper[177], 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. Footnote 177: _Quart. Journ. Micr. Science_, Oct. 1874. [This Edition, No. V.] E and F.