432 DEVELOPMENT OF ELASMOBRANCH FISHES. 



tissue appear around the stalk of the optic vesicle, and in the 

 space between the front end of the alimentary tract and the 

 base of the brain in the angle of the cranial flexure. They are 

 probably budded off from the walls of the head-cavities. Their 

 number rapidly increases, and they soon form an investment 

 surrounding all the organs of the head, and arrange themselves 

 as a layer, between the walls of the roof of the fore and mid- 

 brain and the external skin. At the close of stage K they are 

 still undifferentiated and embryonic, each consisting of a large 

 nucleus surrounded by a very delicate layer of protoplasm pro- 

 duced into numerous thread-like processes. They form a regular 

 meshwork, the spaces of which are filled up by an intercellular 

 fluid. 



I have not worked out the development of the cranial and 

 visceral skeleton ; but this has been made the subject of an 

 investigation by Mr Parker, who is more competent to deal with 

 it than any other living anatomist. His results were in part made 

 known in his lectures before the Royal College of Surgeons 1 , and 

 will be published in full in the Transactions of the Zoological 

 Society. 



All my efforts have hitherto failed to demonstrate any seg- 

 mentation in the mesoblast of the head, other than that in- 

 dicated by the sections of the body-cavity before-mentioned ; 

 but since these, as above stated, must be regarded as equivalent 

 to muscle-plates, any further segmentation of mesoblast could 

 not be anticipated. To this statement the posterior part of the 

 head forms an apparent exception. Not far behind the auditory 

 involution there are visible at the end of period K a few longi- 

 tudinal muscles, forming about three or four muscle-plates, the 

 ventral part of which is wanting. I have not the means of de- 

 ciding whether they properly belong to the head, or may not 

 really be a part of the trunk system of muscles which has, to a 

 certain extent, overlapped the back part of the head, but am 

 inclined to accept the latter view. These cranial muscle-plates 

 are shewn in PI. 15, fig. 15 b, and in PI. 17, fig. 2. 



1 A report of the lectures appeared in Nature, 



