176 



as I count them but that is not in any way essential to the present 

 discussion. 



Fig. 5. Head of an older embryo, showing the anterior accessory vesicle, A. op 1 , 

 occupping the region of the thalamencephalon. op lateral Eye. A. op 1 accessory optic 

 vesicle, mb mid-brain, cb cerebellum, au auditory vesicle. n e , n 7 , etc. neuromeres of 

 hind-brain. He mandibular cavity. 



The facts recorded above are of such a novel nature that it will 

 not be out of place to mention here the precautions I have taken in 

 observing them. The fact that the Zieglek's have recently studied and 

 modeled Torpedo embryos in the same stages of development and have 

 given no record even of the eye vesicles, is against my position, and, 

 of course, there is any amount of negative evidence opposed to it. 



I have had the use of a large number of embryos of Squalus 

 acanthias (and also a few specimens of Galeus canis) covering 

 thoroughly the stages from Balfour's stage "B" to his stage "G". 

 All the stages have been most carefully studied from the exterior and 

 then sectioned. For surface study, I have made it a uniform practice 

 to photograph every specimen studied. I have in addition made a 

 camera sketch of every specimen. The specimens have been rotated 

 and studied from every possible point of view. To avoid being de- 

 ceived by the play of light and shade, I have passed, under the 

 microscope, a beaded bristle into all cavities and over all the surfaces 

 watching carefully its course, and, finally, I have verified the external 

 observations by study of sections. I value most of all, as external 

 views, the series of photographs I have made, since the structures as 



