TELEOSTEI. 



73 



Gotte maintains that Calberla's statements are not to be trusted, and I 

 have myself been unable to confirm them for Teleostei or Lepidosteus; but 

 if they could be accepted the difference in the formation of the medullary 

 canal in Teleostei and in other Vertebrata would become altogether unimpor- 

 tant and consist simply in the fact that the ordinary open medullary groove 

 is in Teleostei obliterated in its inner part by the two sides of the groove 

 coming together. Both layers of epiblast would thus have a share in the 

 formation of the central nervous 

 system ; the epidermic layer 

 giving rise to the lining epithe- 

 lial cells of the central canal, 

 and the nervous layer to the 

 true nervous tissue. 



The separation of the 

 solid nervous system from 

 the epiblast takes place 

 relatively very late ; and, 

 before it has been com- 

 pleted, the first traces of 

 the auditory pits, of the 

 optic vesicles, and of the 

 olfactory pits are visible. 

 The auditory pit arises as 

 a solid thickening of the 

 nervous layer of the epi- 

 blast at its point of junc- 

 tion with the medullary 

 keel ; and the optic vesi- 

 cles spring as solid out- 

 growths from part of the 

 keel itself. The olfactory 

 pits are barely indicated 

 as thickenings of the ner- 

 vous layer of the epiblast. 



FlG. 32. TWO TRANSVERSE SECTIONS OF 



SYNGNATHUS. (After Calberla. ) 



A. Younger stage before the definite es- 

 tablishment of the notochord. 



B. Older stage. 



The epidermic layer of the epiblast is repre- 

 sented in black. 



ep. epidermic layer of epiblast ; me. neural 

 cord ; hy. hypoblast ; me. mesoblast ; ch. noto- 

 chord. 



At this early stage all the 

 organs of special sense are at- 

 tached to a layer continuous 

 with or forming part of the 

 central nervous system ; and 



this fact has led Gotte (No. 63) to speak of a special- sense plate, 

 belonging to the central nervous system and not to the skin, from which 



