VERTEBRATES FROM A CRUSTAOEAN-LIKE ANCESTOR. 427 



The most aulerior part of the substance which fills up the 

 central cavity is composed of masses which are less definite 

 in shape than those near the pigment layer, and are free or 

 nearly free from any appearance of possessing cuticular rods ; 

 they often closely resemble fig. 23, PI. XXVIII, which is 

 taken from the paper of Lankester and Bourne (21), and repre- 

 sents a transverse section of a clump of nerve-end cells from 

 the eye of Euscorpius beyond the region of rods. When 

 stained with picro-boro-carmine it is seen (fig. 24, PI. XXVIII) 

 that the middle portion of these polygonal masses stains red- 

 dish, while the edges stain yellow. 



3. The Cells of the Hypodermal or Vitreous Layer. 

 — The anterior part of the eye is free from pigment, and is 

 composed, as is seen in haematoxylin or carmine specimens 

 (figs, 26, 27), of an inner layer of nuclei which are frequently 

 arranged in a wavy line. These nuclei are continous laterally 

 with the nuclei at the base of the layer of nerve-end cells. 

 From this nucleated layer strands of tissue, free from nuclei, 

 pass to the anterior edge of the eye. 



In the horizontal longitudinal sections it is seen (figs. 26, 

 27) that these strands are confined to the middle of the eye ; on 

 each side of them the nuclear layer reaches the periphery; so that 

 if we consider these strands to represent long cylindrical cells, 

 as described by Beard (20, p. 62), then the anterior wall 

 may be described as consisting of long cylindrical cells, which 

 are flanked on each side by shorter cells of a similar kind. 

 The nuclei at the base of these cylindrical cells are not all 

 alike. We see, in the first place, large nuclei resembling the 

 large nuclei belonging to the nerve-end cells ; these are the 

 nuclei of the long cylindrical cells. We see also smaller nuclei 

 in among these larger ones, which look like nuclei of intrusive 

 connective tissue, or may, perhaps, form a distinct layer of 

 cells, situated between the cells of the anterior wall and the 

 terminations of the nerve-end cells already referred to. The 

 appearance presented is drawn as accurately as possible by 

 Mr. Wilson, of the Scientific Instrument Company, in fig. 27, 

 PI. XXVIII, from a specimen stained with picro-boro-carmine. 



