THE SKELETON OF THE VERTEBRATE HEAD. 91 



the determination of the segments of the embryo or adult 

 head. It appears to me that the segmented structure of the 

 brain is to be looked for, not in its greater masses those 

 developments on its upper surface in which it differs from 

 the spinal cord, and by the possession of which it becomes a 

 brain but in the series of groups of ganglion-cells, the 

 nervous centres of the cerebral nerves, whatever the typical 

 number of these may be, arranged along its base, and strictly 

 homologous with the groups of ganglion-cells which un- 

 doubtedly constitute the morphological segments of the 

 spinal cord. 



The " visceral " or " branchial laminse " afford, in the pre- 

 sent state of the subject, a more secure embryological basis 

 for the determination of the segments of the head. The so- 

 called " first visceral lamina/' the one in which the mandi- 

 bular arch., is developed, and the two succeeding " visceral 

 laminse," those in which the anterior and posterior segments 

 of the hyoid of mammals and birds are formed, must be looked 

 upon as embryological indications of three cephalic segments. 



On the under-surface of the forepart of the embryo head, 

 in front of the so-called " first visceral lamina," there are five 

 processes, in which are developed the palate and pterygoid, 

 the maxillary, malar, and lachrymal, the intermaxillary and 

 nasal bones. The first of these processes on each side extends 

 obliquely forwards from the " first visceral lamina " towards 

 and under the eye. It is the so-called " superior maxillary 

 lobe." The second process on each side the " lateral frontal 

 process" of Eeichert passes down in front of the eye, the 

 eye being situated in the cleft between it and the former 

 process. The fifth process is situated in front, and in the 

 median line. It is the " anterior frontal process " of Eeichert. 

 The clefts or notches between this process and the "lateral 

 frontal process " are considered by Eathke and Eeichert to be 

 the external nostrils. 



