88 



THE AMERICAN NATURALIST 



[Vol. XL 



anterior brain wall is subject to the evagination process, being 

 carried out with the cells of the recessus opticus. 



My conclusions with reference to the eye of amphioxus were 

 based upon a very extensive study of the eye of both the old and 

 the young of amphioxus, and I was able to show that there is a 

 great diversity in the form of the pigment area in different mem- 

 bers of a series of individuals and that there is a tendency for the 

 pigment area to divide symmetrically on either side of the median 



Of greatest moment, therefore, are my observations and those 

 of Kupffer which show that the pigment to be later used in the 

 retina of the eye is first of all laid down in the inner ends of the 

 cells of a primitively unpaired, even though bilaterally symmetrical, 

 plate of cells which evaginates from the brain as the recessus opticus. 



Minot attempts to homologize the vertebrate eye and optic tract 

 with the highly differentiated arthropod eye, supra-esophageal gan- 

 glion and the circumesophageal nerve ring, but the idea that the 

 visual organ of the vertebrates is to be sought for in such a spe- 

 cialized organ as the compound eye of arthropods is unsupported 

 by morphological facts. 



The nose in amphioxus remains in the form of a conical epithe- 

 lial pit, whose apex is connected with or is in contact with the 

 anterior end of the brain. 



This pit is the so called sinus olfactorius impar, being the 

 remains of the anterior neuropore. The right and left walls of 

 this conical pit are thus morphologically equal, and, notwith- 

 standing the fact that the pit is later pushed to one side by the 

 growth of the base of the median fin-fold, we must hold that it 

 is bilaterally symmetrical both in origin and in adult life and is 

 strictly comparable to the plate of cells which evaginates from 

 the anterior end of the brain of Bdellostoma and of the sturgeon, 

 and which has been conveniently called the unpaired nasal plate. 



It has long been accepted that the nasal epithelium of the 

 Gnathostome vertebrate is laid down as a pair of bilaterally sym- 

 metrical plates in the embryo and continues paired throughout 

 life, while in the Cyclostome it is laid down in an un[)aired con- 

 dition and ever remains so. Xothing could l)e more incorrect, 

 for the plate in some Amphibia is identical with that in the Cyclo- 



