74 RICHARD EVANS. 



tlie constriction of the wide stalk shown in Fig. 25 h a cord 

 of white matter has developed inside it. The cord in ques- 

 tion passes from the white matter of the brain to the eye, 

 and seems to spread itself over the inner half, the nuclei of 

 which are arranged in many layers (PI. 9, fig. 25 c). There 

 seems to be no doubt that this cord, that is the optic nerve, 

 is developed in situ from the wide stalk of earlier stages 

 (PI. 9, fig. 25 &), much in the same way as the nervous strands 

 which connect the ventral organs to the nerve-cords are 

 formed, but with one important difference, namely, that the 

 cells which form the latter are scarcely modified as compared 

 with those of the grey matter of the nerve-cord; while the 

 cells which give rise to the former, that is to the optic nerve, 

 undergo changes similar to those brought about in the ele- 

 ments which produce the white matter of the brain and 

 nerve- cords. The cells which give rise to the white matter 

 are developed from the ordinary cells of the sensory rudi- 

 ment, that is the common rudiment of the nervous system, 

 eyes, and ventral organs. The cells of the grey matter have 

 small and highly granular nuclei, and a very small amount 

 of cytoplasm ; but the cells which give rise to the white 

 matter have large nuclei, with very fine chromatin granules 

 and a large amount of cytoplasm, in which there are no 

 distinct cell outlines. The cells producing the white matter 

 were only observed in embryos. I am not aware of their 

 having been described before. 



Conclusion. — There seems to be no doubt as to the cor- 

 rectness of Mr. Sedgwick's account of tlie development of 

 the eye in P. capensis, as a cerebral eye, and the same 

 statement is equally true of the formation of that of E. 

 weldoni. 



VIII. The Endoderm. 



The history of the younger stages in the development of 

 the endoderm has been given in the early part of this paper 

 (pp. 40 — 55), but owing to the importance of the question it 

 is necessary to recapitulate, and to add some more facts re- 



