No. 3-] THE VERTEBRATE HEAD. 559 



These accessory structures are present for a brief time only. 

 That region of the head, behind the optic vesicles and in front 

 of the ears, becomes the seat of great modifications, and, as 

 the neural groove closes, the structures described give way to 

 later formed ones. They are, therefore, not only embryonic 

 structures, but are also transitory. It is a truism in develop- 

 mental history that the more primitive characteristics appear 

 first, and the secondary modifications come in later. The 

 structures described are among the most primitive that have 

 been preserved in this very ancient group of Vertebrates, and 

 should be of significance in indicating the ancestral relations 

 of the eye. I have spoken of the disappearance of the organs, 

 but I have been able to trace the anterior pair further along, 

 an account of which will be given in the next section on The 

 Pineal Sense-Organs. 



I have also traced out similar rudimentary organs in the 

 embryo chick, at about the conventional 24-hour stage ; there 

 is a series of seven vesicles behind the optic vesicles. These 

 structures are rudimentary, and very quickly fade away. 



All this acquires new interest when taken in connection 

 with the researches of Whitman on the eyes of leeches. As 

 indicated above, he showed that the eyes of the leeches are 

 derived from segmental sensory papillae. It now appears that 

 we have traces of a somewhat similar line of segmental sensory 

 organs in Vertebrates. It is essential to say traces, for these 

 structures are transitory, and do not rise to a high grade of 

 differentiation, except in the case of the pineal sense-organs. 

 They may never, in the vertebrate group, have been raised to 

 the rank of eyes, but it is certain that in Annelids similar 

 segmental sensory patches have been so raised. It is, however, 

 reasonable to suppose that members of this series have been 

 functional, even in Vertebrates, when we recollect the highly 

 developed condition of the pineal eye in some forms. 



There is really very little direct evidence to support the 

 proposition that the Vertebrates are derived from multiple-eyed 

 ancestors, although it is a conception that has been in the 

 minds of morphologists for some time. Besides the indications 

 of such a condition afforded by the facts of this paper there 



