No. 3-] THE VERTEBRATE HEAD. 549 



I. The Lateral Eyes. 



Squalus acanthias is an especially favorable form for observ- 

 ing the beginnings of the optic vesicles. They make their 

 appearance in this animal at a very early stage, while the neural 

 plate is broadly expanded, and before the neural folds have 

 arched upwards in any part of the embryo. I should estimate 

 their appearance in this animal to be as early as in any other 

 animal in which they have been described. Previous to 1889 

 it had been current opinion that the optic vesicles appeared 

 very early only in the class of Mammals. Bischoff, Kolliker, 

 His, Van Beneden, Heape, and others had recorded their early 

 appearance in that group ; but it was regarded as a precocious 

 development for some reason confined to mammalian forms. 



More careful observations on the earlier stages have 

 shown that the optic vesicles customarily arise in different 

 animals much earlier than was supposed. In 1889 Whitman 

 called attention to the very early appearance of the optic vesi- 

 cles in Necturus: "Its basis being discernible as a circular 

 area — after treatment with osmic acid, followed by Merkel's 

 fluid — long before the closure of the neural folds of the brain." 

 Dural noted the early appearance in birds. In 1 893 Eycleshymer 

 gave notes upon their appearance and structure in Necturus 

 and other Amphibia. His most noteworthy observations are 

 upon Rana palustris, a form hitherto unstudied, in which he 

 finds a remarkably early development of the primary optic vesi- 

 cles. They are very evident from surface study in this form, 

 on account of the distribution of pigment granules in them, just 

 after the neural ridges have begun to form. Cross-sections in 

 these early stages show a considerable amount of histological 

 differentiation from the surrounding cells. 



The early differentiation of the optic vesicles had not been 

 recorded in any elasmobranch form until the publication of my 

 preliminary papers in 1893 and 1894, where the optic vesicles 

 are described and their serial relation to other structures on 

 the cephalic plate is shown. Since then I have had opportu- 

 nity to confirm my observations on Squalus, and to extend them 

 to some other forms ; for instance, in Diemyctylus and 



