THE MESENCEPHALON. 



67 



embryo (frog), it will be seen that the underlying neuromeres form a natural 

 group, that corresponds approximately with the posterior four or five thoracic 

 neuromeres of arachnids. It is impossible, at present, to accurately fix either the 

 anterior or the posterior boundaries of this group, for we do not know how many, 

 if any, have been added to the cheliceral neuromere to form the diencephalon, or 

 whether or no the last, or sixth, thoracic neuromere has fused with the first vagus. 

 There should be, according to our provisional interpretation, five neuromeres, 

 from the fifth to the ninth inclusive, in this region; and that number corresponds 

 pretty nearly with the estimated number of neuromeres, and with the number 

 of head cavities, segmental nerves, and visceral arches that are known to occur 

 there. (Fig. 58.) 



IV. The Metencephalon, or Vagus Neuromeres, and V, The Branchien- 



CEPHALON, OR BRANCHIAL NeUROMERES. 



The Metencephalon. — In many arthropods, the transitional region between 

 one group of metameres and the next is often marked by an abrupt change 

 in the size of the pertinent organs, and by their union in the middle line and 



Fig. 55. — Section through the first vagus neuromere of Limulus, showing the neural and haemal commissures, central 



canal, and lemmatochord. 



ultimate disappearance; at the same time, in the next following segments, the same 

 kind of organs may be greatly enlarged. These conditions are similar to those 

 seen at the point of incomplete fissure in annelids. 



The most striking cleavage zone of this character in the arthropods is the 

 vagus region. It lies between the thorax and abdomen, and is one of the most 

 conspicuous transitional zones in the whole body. It is present in many insects, 

 Crustacea, trilobites, etc., but its character is best known in the arachnids. 



In Limulus, there are two vagus metameres, the chilarial and opercular, 

 whose neuromeres form an important part of the brain. The external boundaries 

 of the two metameres are at first similar to those of the other abdominal segments. 

 (Figs. 141, 142.) But ultimately, the only remaining external traces of the chil- 

 arial segment are the chilarial appendages and the narrow ridge on the posterior 



