THE METACEPHALON. 1 9 



vertebrates, as the opening behind the cerebellum, now closed by the choroid 

 plexus of the fourth ventricle. (Figs. 3, 43, 44, 46, 58.) 



The mesencephalon consists of the last three or four thoracic neuromeres; 

 they are usually conspicuous for their distinctness, great breadth and volume, 

 and for the large size of their ganglia. In the vertebrates, they form the pos- 

 terior portion of the crura cerebri, and are still further accentuated as one of the 

 principal divisions of the brain, by the migration of the optic ganglia of the lateral 

 eyes backward and upward till they come to overlie them as the tectum opticum. 

 The parietal eye ganglia overlie the diencephalon as the ganglia habenulae. (Figs. 



43> 44, 57, 5 8 -) 



The Suprastomodceal Commissure and the Cerebellum. In all arthropods, 

 the lateral stomodaeal ganglia are united by a large commissure that forms a 

 prominent arch over the anterior or neural surface of the stomodaeum. This com- 

 missure is one of the most conspicuous and constant landmarks in the arthropod 

 brain. (Fig. 3, st.co.) In the insects, it contains a large, median mass of gan- 

 glion cells, arising as an evagination, or as a thickening, in the anterior, median 

 wall of the stomodaeum, close to its external opening. (Fig. 3, a.) The projecting 

 arch of the commissure becomes crowded backward by the backward migration 

 of the mouth and rostrum, and by the increasing size of the lateral eye ganglia, 

 forming in vertebrates the rudiment of the cerebellum. (Figs. 3, D, and 46.) 



Thus the median stomodaeal ganglion of arthropods and the cerebellum of 

 vertebrates are the only brain structures that may be said to arise originally in the 

 median line above the neural surface of the brain; the parietal eyes, the ganglia 

 habenulae, and the optic lobes being originally paired structures arising from the 

 lateral margins of the medullary plate. 



4. The Metacephalon, or Vagus Region. 



The metacephalon, or vagus region, forms a remarkable intermediate zone 

 between the mesocephalon and the branchiocephalon. It consists of from one 

 to four metameres that usually atrophy, or fuse with one another at an early 

 period, leaving little or no external trace of their existence in the adult. Their 

 feeble development is the principal cause of the sharp constriction which, in many 

 insects and arachnids, separates the thorax from the abdomen. (Figs. 3, 6, 14, 

 15, 16, 46, 47, 57, M. c. or I'g. 1 " 4 .) 



The vagus appendages rarely serve as locomotor or respiratory organs. They 

 show a marked tendency to become unpaired; they may dwindle into insignifi- 

 cance, or they may be retained as highly specialized sense organs (chilaria and 

 metastoma of merostomes; genital papillae and pectens of scorpions). 



The vagus neuromeres, on the other hand, are well-developed, but they fuse 

 with one another so quickly that it is very difficult to distinguish their boundaries 

 after the early embryonic stages. Their motor elements are greatly reduced and 

 the sensory ones correspondingly enlarged, owing to the reduction of the corre- 



