DEVELOPMENT OF ELASMOBEANCH FISHES. 209 



tissue appear around the stalk of tlie 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 in- 

 vestigation 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 \ and will be published in full in the Transactions of 

 the Zoological Society. 



All my efforts have hitherto failed to demonstrate any 

 segmentation in the mesoblast of the head, other than that indi- 

 cated 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 audi- 

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

 longitudinal muscles, forming about three or four muscle-plates, 

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

 of deciding 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. xiv. fig. 15 6, and in PI. xvi. fig. 2. 



Notochord in the Head. 

 The notochord during stage G is situated for its whole length 

 1 A report of the lectures appeared in Nature. 



