No. 2.] VERTEBRATE CETHALOGEJVESIS. 229 



pigment spot assumes a large variety of shapes as well as posi- 

 tions, with respect to the anterior wall of the ventricle and to 

 the first pair of cranial nerves. The most usual form is that of 

 a slightly bilobed mass ; the lobes being placed to the right and 

 left of the median line, so as to cover the roots of the first pair 

 of cranial nerves more or less completely. 



Other forms have been described by the various authors who 

 have written upon the subject, and they are easily observable 

 in any scries of individuals. As already stated, the spot is very 

 variable in size and shape, and its extremes may be set, as the 

 convex or concave (forwards) lens shape, and the close group- 

 ing of most of the pigment cells in the wall of the lamina ter- 

 minalis on the one hand, and the separation of the mass into 

 three portions (two ventral and lateral, and one dorsal and me- 

 dian) with outlying scattered pigment cells on the other hand. 

 These various forms grade insensibly into each other, and 

 evidently depend in large degree, if not entirely, upon the migra- 

 tory capabilities of the pigment cells in a state of nature. 



These variations, then, are caused by the shifting of the pig- 

 ment cells. It may be frequently observed that the two pig- 

 ment masses pass out into the two nerves of the first pair. 

 Where this modification occurs, it will be found associated with 

 a pair of diverticula of the median ventricle which make their 

 way out into the nerve roots. These diverticula are lined by 

 the same cells that form the inner lining of the median ven- 

 tricle, and the pigment cells lie among the cells of the second 

 layer, or layer of percipient elements. 



In this manner the light-perceiving organs are carried toward 

 the surface at the ends of diverticula of the primitive brain 

 cavity, reproducing in the phylogeny of Amphioxus the first 

 steps of the series known for the remaining vertebrates. 



We have only to think of these two pigmented optic struc- 

 tures brought into relation with the ectodermic structures — 

 lens, cornea, etc., — of the higher vertebrate eye, in order to 

 have all the steps of eye production carried out in Amphioxus. 

 We know of no such relation of the ectoderm in the Amphi- 

 oxus, and consequently conclude that, all facts considered, Am- 

 phioxus is descended from some ancestral form along with other 

 vertebrates, but that, owing to its simplie habits of life, has 

 never required a lens or other focusing apparatus for the pur- 



