36 OBSERVATIONS ON THE DEVELOPMENT OF RAIA BATIS. 



eventually the pectorals gain the foremost part of the side of the head, at the same 

 time becoming united with the extended facial disk. 



The tail, as the whole animal increases in size, becomes relatively very much shorter. 



* 



In the earlier stages the body is only three sixteenths of the entire length of the 

 embryo, but subsequently it is about one half that length, as will be seen by a 



comparison of figs. 7, 8, 9, and 11. 



Mouth. — In the youngest embryos this has an elongated shape, is broadest at the 

 fore part, and, contrary to what is usually the case in the embryos of other orders, has 

 its longest diameter directed from before backwards, which circumstance gives it a 

 very singular appearance (fig. 4 a ). Its borders are formed by the first branchial 

 arch (figs. 4 a and 5, b), and, as development advances, its longest diameter begins 

 to shorten, and the arch bends on either side, its upper and lower, or its fore and 

 hinder portions, forming an angle with each other ; and thus begin the future angles 

 of the mouth. That portion of the arch below these is without a doubt transformed 

 into the lower jaw, and the " upper jaw " is formed of what remains in front of them. 



The homology of this so-called upper jaw has led to much discussion. One thing 

 is certain, there is at no time to be seen in the circumference of the mouth anything 

 which corresponds with the "intermaxillary bud" or " fronto-nasal protuberance"; 

 nor is there anything corresponding with an upper " maxillary bud." The only other 

 parts to which the " upper jaw " could be said to correspond would be the palatines, 

 as asserted by Cuvier; or more probably, as maintained by Mr. Huxley, the bones 

 just mentioned, the pterygoids and quadrate bones together, all of which are believed 

 to be developed from one and the same primary cartil 



If the maxillary and intermaxillary bones exist at 



& 



& 



exist at all in the head of the skate, their 

 homologues must be found in the parts farther forward than those just mentioned, 

 and in some way connected with the nostrils. We shall, therefore, speak of them 

 when these last-named parts are described. 



There is a very important change, which, though not directly connected with the 

 mouth, yet involves the region of it, and may properly be mentioned here ; namely, 

 the formation, just in front of the nostrils, across the whole breadth of the under side 

 of the head from one pectoral fin to the other, of a ridge of thickened integument, 

 which gradually extends forward in a horizontal plane, forming what may be called 

 the facial disk, and is most prominent in the middle (figs. 7, 9, and 10, b). It is by 

 the extension of this forward till it passes beyond the foremost part of the cranium, 

 and its fusion with the pectorals, that the pointed rostrum of the adult skate is 

 formed (fig. 11, e). Even after this is nearly completed, the cranium remains promi- 

 nent above it, but eventually both come to the same level 



