SOCIETY OF THE UNIVERSITY OP ABERDEEN. 109 



not reach the ventral margin of the brain, while the second neuromere 

 is the largest and best denned. The third neuromere is also prominent, 

 but not so wide as the second, and its ventral end tapers oft' to a 

 point between the ventral ends of the second and fourth neuromeres. 

 The fourth neuromere is pointed dorsally and widens out below, 

 between its ventral end and that of the second neuromere being a 

 notch as described by Bremer (2) in a 4 mm. embryo. The ganglion 

 acustico-facialis is attached to the upper end of this neuromere, but 

 largely lies in the furrow between the third and fourth neuromeres. 

 The fifth, sixth and seventh are not so clearly marked off. 



The oval auditory field of thickened ectoderm lies over the dorsal 

 end of the fifth neuromere, but also extends over the upper end of the 

 fourth. The trigeminal ganglion is not developed. The ganglion 

 acustico-facialis is large, and is attached to the lateral wall of the 

 hind-brain in connection with the anterior margin of the fourth 

 neuromere. The ganglion of the vagus is only being developed. 



The flexures of the brain agree very closely with the flexures 

 described by Professor Thompson as being present in his embryo of 

 twenty-three somites. There is a prominent flexure in the region 

 between the mid-brain and hind-brain, the mid-brain and fore-brain 

 being bent downward. The anterior extremity of the notochord is 

 bent down in a corresponding manner. There is also a distinct 

 flexure of the hind-brain at the notch between the ventral ends of the 

 second and fourth neuromeres. The neck flexure is just commencing, 

 and forms an open angle (Plate XIII.). 



In conclusion, I wish to express my thanks to Professor Wieders- 

 heim for kindly granting me permission to work in his laboratory ; 

 more especially to Professor Keibel for the loan of the embryo, and 

 much help and advice ; and also to Professor Pfannenstiel to whom 

 the embryo belongs. I also express my indebtedness to Professor 

 Keid in connection with my research work, and to the Carnegie 

 Trustees for a grant toward the cost of embryological research and the 

 illustration of this paper. 



