8 4 



THE ORIGIN OF VERTEBRATES 



I 



as already indicated, Patten looks on all these eyes as three-layered, 

 and states that in all cases a corneagen or vitreogen layer exists, 

 which gives origin to the lens. For my own part I agree with 



Patten, but we are not con- 

 cerned here with the lateral 

 eyes. It is sufficient to note 

 that all observers are agreed 

 that the median eyes are 

 characterized by this well - 

 marked cell-layer, the so-called 

 vitreous or corneagen layer of 

 cells. 



This layer (p., Fig. 35) is 

 composed of much - elongated 

 cells of the hypodermal layer, 

 in each of which the large 

 nucleus is always situated to- 

 wards the base of the cell. 

 The space between it and the 



Fig. 36. — Eye op Hydrophilus Larva, 

 with the Pigment over the Retinal 

 End-cells. 



retina contains, according to 



I., chitinous lens; c, corneagen; pr., pre- 

 retinal layer ; rh., rhabdites; ret., retinal 



end-cells. 



Patten the cells of the pre- 

 retinal layer (pr.). These may be so few and insignificant as to give 

 the impression that the vitreous layer is immediately adjacent to the 

 retina (ret.). 



Let us turn now to the right pineal eye of Ammoccetes (Fig. 37) 

 and see what its further structure is. The anterior part of the eye 

 is free from pigment, and is composed, as is seen in hsematoxylin or 

 carmine specimens, of an inner layer of nuclei which are frequently 

 arranged in a wavy line. 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 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, 

 then the anterior wall may be described as consisting of long 

 cylindrical cells, which are flanked on either 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 



