VERTEBEATES FROM A ORUSTAOEAN-LIKE ANCESTOR. 425 



nothing except the coagulated remnants of an albuminous 

 fluid. In none of my specimens^ whether cut horizontally or 

 transversely, is there any sign of such a central cavity as figured 

 by Beard ; in all cases this space is filled with the remains of 

 what is clearly a tissue possessing a well-defined structure. In 

 the older Ammocoetes the substance of this tissue is very apt 

 to be fissured irregularly (as in fig. 20, PI. XXVII), or else to 

 be vacuolated, the spaces so formed being of different sizes 

 and somewhat circular in outline. 



In all cases, however, square-shaped blocks and clumps of 

 tissue, in which there is no sign of nuclei, are seen lying 

 between the pigmented posterior part of the eye and the 

 anterior non-pigmented part; and it is significant, as showing 

 how this vacuolated appearance has misled Beard, that the 

 younger the animal the more regular and compact is the 

 arrangement of these square- shaped masses of tissue. Accord- 

 ing to Shipley (10), the eye at its commencement possesses no 

 sign of an internal cavity, but presents an appearance of a regu- 

 lar solid mass of cells ; this statement is denied by Owsjannikow 

 (7), who says that the anterior and posterior walls come so 

 close together at an early stage of development as to almost 

 obliterate the appearance of a central cavity. Everything 

 seems to me to point to the conclusion that the appearance 

 of a large central cavity is brought about by the partial de- 

 generation of elements which originally filled it, and that their 

 remains have given rise to the impression held by Spencer 

 and Beard, that a large cavity exists which is filled with a 

 coagulated albuminous fluid. 



In addition, it may be remarked that it is perfectly easy to 

 see the appearance presented by the coagulated albuminous 

 fluids within an eye by simply looking at sections of the lateral 

 eyes of the same animal ; the appearance presented resembles 

 that of a blood-clot, and is totally dissimilar to the square- 

 shaped masses of protoplasmic-looking material seen in the 

 cavity of the pineal eye. 



The examination of these square-shaped masses by a high 

 power in osmic preparations, especially in the neighbourhood 



