152 Edward Phelps Allis jr., 



line of fusion of the ethmoidal and fronto-sphenoidal elements of the 

 skull. In my 80 mm specimen the anterior or ethmoidal part of the 

 canal definitely ends at this line, a second, post-ethmoidal central canal 

 beginning-, immediately, in the fronto-sphenoid. The wall that here 

 definitely separates these two canals or cavities may, accordingly, dis- 

 appear in the adult. 



About midway between the sensory ethmoidal chamber and the 

 anterior end of the interorbital opening^ the floor of the large ventral 

 canal or cavity above described is pierced in the middle line, and a 

 small, median, and still more ventral canal leads from here into the 

 extreme anterior end of the interorbital opening, immediately above 

 the floor of the opening. This canal transmits a bloodvessel which 

 has a large branch extending forward to the sensory ethmoidal 

 chamber and a small one extending backward into the posterior 

 portion of the large central cavity. These bloodvessels were only 

 traced in my 80 mm specimen, in which specimen they were changed 

 with blood corpuscles. The vessel, as it traverses the median opening 

 in the floor of the central cavity and then the small median canal 

 that leads to the interorbital opening, is small and median in position. 

 When it reaches the interorbital opening it turns to the left (in the 

 sections) and soon comes to lie against the dorso-lateral aspect of the 

 olfactory nerve of that side. This nerve, as it traverses the inter- 

 orbital region, comes to lie directly dorsal to the nerve of the op- 

 posite side. Each olfactory nerve is accompanied by numerous blood- 

 vessels, which come from the region of the nasal epithelium, and are 

 exactly similar in appearance, in sections, to the median bloodvessel 

 here under consideration. As the two olfactory nerves acquire a 

 position one dorsal to the other, the bloodvessels accompanying each 

 nerve collect on what was the mesial side of the nerve they ac- 

 company, and intercommunicate, but do not fuse to form a single 

 vessel. Fig. 19 shows the arrangement in this region, the vessels 

 accompanying each nerve forming what seems to be a retiform struc- 

 ture. As the olfactory neri^es now pierce the cranial membranes to 

 enter the cranial cavity, the related vessels do not enter the cavity 

 with them, but pass to the right and left, the right vessel (in sections) 



