No. 2.] VERTEBRATE CEFHALOGENESIS. 233 



That these differences are due to primitiveness of the organs 

 in Amphioxus, and not to any inherent fundamental differ- 

 ences between the eyes of Amphioxus and the corresponding 

 parts in other vertebrates, will be apparent to any one who will 

 analyze and compare the earliest stages in the development of 

 the eye in the several vertebrate groups. 



We cannot believe that the paired eyes of vertebrates, as 

 they are known from the Cyclostomes upward, sprang into 

 existence with complete optic vesicles, optic stalks and nerves. 

 Much less, that they were provided from the first with lens and 

 cornea ; on the contrary, the facts of ontogeny, the existence of 

 such an idea as that of evolution in the domain of biological 

 science, compel us to assume that the process was a gradual 

 one and lasted through a long period of time, progressing only 

 by minute increments, beginning with such a rudiment as I have 

 shown to exist in Amphioxus. With the increase in complexity 

 of organization of the type and specialization of the functions of 

 the sense organs, came the added increments, which have re- 

 sulted in the very complex, and in many ways still unknown, 

 structure and relations of the paired and median eyes. 



It is further unavoidable, that we conclude that the rudiments 

 of eyes were functional from the very first or incipient stages 

 through each modification up to the completed form, and that 

 the physiological function of the organs in question have under- 

 gone as extensive, as important, and in every way a parallel 

 growth, by slight increments acquired with the added morpho- 

 logical increments. 



To those morphologists who object to the view I have thus 

 briefly stated and endeavored to support, that such simple diver- 

 ticula as are found in Amphioxus associated with pigment spots 

 are entirely inadequate to give rise to such complicated struct- 

 ures as the eyes, I wish to give answer here : that we have in 

 the case of the extremely complicated organ of the mammalian 

 ear, an organ which has originated in a very similar manner, 

 and whose every stage of development may be traced in existing 

 vertebrates and about which morphologists are agreed that it is 

 formed by simple involution of primitively superficial sense 

 organs. The development of the cochlea, and its contained 

 structures, is another instance of the production of an extremely 

 complicated organ from a simple diverticulum pushed out from a 



