DEVELOPMENT OF THE ELASMOBRANCH FISHES. 87 



The embryo does not take more than twenty-four hours in 

 passing from this stage, when the head is a flat plate, to the 

 stage when the whole neural canal (including the region of 

 the head) is closed in. The other changes, in addition to the 

 closing in of the neural canal, are therefore somewhat insig- 

 nificant. The folding off of the embryo from the germ has, 

 however, progressed considerably, and a portion of the hind 

 gut is closed in below. This is accomplished, not by a tail-fold, 

 as in Birds, but by two lateral folds, which cause the sides of 

 the body to meet and coalesce below. At the extreme hind 

 end, where the epiblast is continuous with the hypoblast, the 

 lateral folds turn round, so to speak, and become continuous 

 with the medullary folds, so that when the various folds meet 

 each other an uninterrupted canal is found passing round from 

 the neural into the alimentary canal, and placing these two in 

 communication at the tail end of the body. Since I have 

 already mentioned this, and spoken of its significance, I will not 

 dwell on it further here. 



The cranial flexure commences coincidently with the closing 

 in of the neural canal in the region of the brain, and the divi- 

 sion into fore, mid, and hind brain becomes visible at the same 

 time as or even before the closing of the canal occurs. The 

 embryo has now become more or less transparent, and proto- 

 vertebrae, of which about twenty are present, can now be 

 seen in the fresh specimens. The heart, however, is not yet 

 formed. 



Up to this period, a period at which the embryo becomes 

 very similar in external appearance to any other vertebrate em- 

 bryo, I have followed in my description a chronological order. 

 I shall now cease to do so, since it would be too long for a pre- 

 liminary notice of this kind, but shall confine myself to the 

 history of a few organs whose development is either more im- 

 portant or more peculiar than that of the others. 



I have thought it worth while to give a short history of 

 the development of the protovertebrae, firstly, because it is 

 very easy to follow this in the Dog-fish, and, secondly, because 



