146 BIOLOGICAL LECTURES. 



Within the last few. years another test of segmentation has 

 been adduced, that of the segments (neuromeres or encephalo- 

 meres) of the brain. These structures had been noticed by 

 several of the older writers upon the development of the ner- 

 vous system, but little weight was given them until Kupffer 

 brought them prominently into notice as possibly affording 

 another clue to the segmentation of the head. The idea was 

 further carried out by Beranek, Orr, Waters, McClure, Zimmer- 

 mann, and others, and may be stated in its present form some- 

 what as follows. Besides the division of the brain into its 

 several regions, fore-brain, 'twixt-brain, etc., this structure 

 shows in its earlier stages another segmentation, most plainly 

 seen in its lateral walls. For instance, the medullary region is 

 seen to consist laterally of a series of paired enlargements, sep- 

 arated by vertical constrictions, and of these back to the vagus 

 there are six (Orr) or seven (Hoffmann). These neuromeres 

 bear a definite relation to the nerves, one pair of these arising 

 from each neuromere, except that between the acusticofacialis 

 and glossopharyngeal. 1 In the other brain-regions there are 

 four more of these enlargements, two in the primitive fore-brain 

 and two in the mid-brain, a total of eleven back to and includ- 

 ing the vagus. 



Hoffmann has made a most important discovery in connection 

 with the development of the cranial nerves. He finds that the 

 segmental head nerves and the dorsal roots of the spinal nerve 

 arise not from a solid neural crest, but are paired segmental 

 out-pocketings (Ausstulpungen) of the dorsal part of the ner- 

 vous cord itself. For the details of the matter reference must 

 be made to Hoffmann's paper, but we may point out that a most 

 important inference is to be drawn from this account. Briefly, 

 then, the segmental cranial nerves (trigeminal, glossopharyngeal, 

 etc.) arise as hollow outgrowths from the wall of the cerebral 

 tube ; these outgrowths reach the skin of the sides of the head, 

 fuse with it, and from the thickening thus produced the ganglion 

 of the nerve is formed. From this ganglion the permanent 

 nerve grows back to the brain, the primary nerve disappearing. 



1 Hoffmann finds an Anlage of a nerve here at an early stage. It later aborts, 

 and cannot be regarded as forming a part of the auditory nerve. 



