No. 3-] THE VERTEBRATE HEAD. 567 



that there exists in my mind some doubt as to the adequacy 

 of the above account. It may have to be modified, but I have 

 given the facts as they now appear to me. 



Figs. 38-54 show a series of embryos that have been partly 

 dissected in the way just indicated. They show characteristic 

 changes in the brain-walls as seen from the exterior. 



In Fig. 38 the walls of the neural groove have not yet met 

 in any part of their course, and the thalamencephalon cannot 

 be distinguished from the rest of the brain. The first pair of 

 accessory vesicles are visible; they soon become incorporated 

 in the walls of the thalamencephalon. I have not been able 

 to determine whether it is the epithelium of the first pair of 

 accessory vesicles only, or whether epithelium from the second 

 pair is also included. Either the epithelium of the two pairs 

 is incorporated into the walls of the thalamencephalon by 

 being carried together, or the epithelium of the second pair 

 fades into the surrounding substance of the brain-wall, which 

 almost immediately grows into the vesicle of the mid-brain. 

 Although I am doubtful as to whether the second pair of ac- 

 cessory vesicles pass into the inter-brain, I am sure that the 

 epithelium of the anterior pair is so included. 



Fig. 39 shows the first accessory optic vesicle and the mid- 

 brain vesicle forming a common protuberance, but they are 

 very quickly separated, and the anterior vesicle goes into the 

 thalamencephalon. 



In Fig. 40, which is somewhat older, this has occurred. The 

 lateral walls of the thalamencephalon now consist of two shal- 

 low cups, approximated so that the structure looks lenticular 

 when viewed from above. As a usual thing, the thalamen- 

 cephalon is not evident at this stage before the removal of the 

 overlying tissues. Fig. 41 shows, however, a specimen in 

 which it could be seen before any dissection. Compare this 

 with Fig. 32, which is the more usual appearance of an embryo 

 of this age. 



Fig. 42 is a dissection of the embryo shown in Fig. 41. 

 The mid-brain, which, from external view, appears like a 

 single rounded eminence, is shown after exposure to be 

 bilobed. 



