Oi)tic Lobes of FoiuL 79 



series of drawings, depicting the external formation and the 

 descent of the optic lobes in relation to the adjacent parts of the 

 brain, has apparently not been taken before — hence the present 

 paper. 



The drawings are divided into 10 sets of 6 each, and an 

 eleventh set, showing the dorsal and lateral view of the adult 

 brain for comparison with the foregoing figures. Each of the 

 ten sets contains two solid drawings, that of the dorsal and 

 lateral views respectively. There are also three sections taken 

 transversely across the optic lobes^ — (1) across the anterior 

 portion, (2) across the middle, and (3) across the posterior 

 portion. The sixth drawing is a diagram of the lateral view 

 of the brain, showing the approximate direction of the above 

 three sections. 



The solid draAvings are taken under the camera lucida. and 

 are as nearly as possible twice their real size. The sections 

 are drawn under a Leitz camera lucida, and 1 x power, Leitz 

 lens, and have been lialved in reproduction. 



The work has been carried on in the Biological Laboratory of 

 the Melbourne University, and I should like to express my 

 thanks to Professor Baldwin Spencer for assistance and advice, 

 and to Messrs. J. Brake, A. O. V. TVmms and A. L. Rossiter 

 for help in various ways. 



Stage I. (PI. XXY.) 



The first stage corresponds approximately with the period of 

 four days' incubation. At this point of development, the mesen- 

 cephalon is represented by a very marked median dorsal swell- 

 ing, which is usually said to correspond to the fourth and fifth 

 neuromeres. A very slight groove (lb) beginning to form in the 

 sagittal plane, and extending right along the dorsal surface of 

 the mesencephalon, is the first indication of the division into 

 the optic lobes. The epiphysis at this time is becoming pro- 

 minent, and the cerebral hemispheres have assumed the dorsally 

 recurved appearance characteristic of their earlier development. 



In la, a lateral view of the brain is shown, with the eye 

 forming. The epiphysis is seen in profile. The great distance 

 between the small cerebral hemispheres, and the large mesence- 



