320 THE ORIGIN OF VERTEBRATES 



This latter gland undoubtedly arises posteriorly to the hypophysial 

 tube, or Eathke's pouch (as it is sometimes called), and, as already 

 mentioned, is supposed by Kupffer to be formed from the posterior 

 wall of this pouch. More recently, as pointed out in Haller's paper, 

 Nusbaum, who has investigated this matter, finds that the glandular 

 hypophysis is not formed from the walls of Eathke's pouch, but from 

 the tissue of the rudimentary connection or stalk between the two 

 premandibular cavities, which becomes closely connected with the 

 posterior wall of Eathke's pouch, and becoming cut off from the 

 rest of the premandibular cavity on each side, becomes permanently 

 a part of the ' Hypophysis Anlage.' 



The importance of Nusbauin's investigation consists in this, that 

 he derives the glandular hypophysis from the connecting stalk 

 between the two premandibular cavities, and therefore from the 

 walls of the ventral continuation of this cavity on each side. 



This may be expressed as follows : — 



The ccelomic cavity, known as the premandibular cavity, divides 

 into a dorsal and a ventral part ; the walls of the dorsal part give 

 origin to the somatic muscles belonging to the oculomotor nerve, 

 while the walls of the ventral part on each side form the connecting 

 stalk between the two cavities, and give origin to the glandular 

 hypophysis. 



Now, as already pointed out, the premandibular cavity is homo- 

 logous with the 2nd prosomatic ccelomic cavity of Limulus, and this 

 2nd prosomatic ccelomic cavity divides, according to Kishinouye, into 

 a dorsal and a ventral part ; and, further, the walls of this ventral 

 part form the coxal gland. Both in the vertebrate, then, and in 

 Limulus, we find a marked glandular tissue in a corresponding 

 position, and the conclusion is forced upon us that the glandular 

 hypophysis was originally the coxal gland of the invertebrate an- 

 cestor. As in all other cases already considered, when the facts of 

 topographical anatomy, of morphology and of embryology, all com- 

 bine to the same conclusion as to the derivation of the vertebrate 

 organ from that of the invertebrate, then there must be also a struc- 

 tural similarity between the two. What, then, is the nature of the 

 coxal gland in the scorpions and Limulus ? Lankester's paper gives 

 us full information on this point as far as the scorpion and Limulus 

 are concerned, and he shows that the coxal gland of Limulus differs 

 markedly from that of Scorpio in the size of the cells and in the 



