TELEOSTEI. 



59 



a 



- 



altogether unimportant 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 epithelial cells of the central 

 canal, and the nervous layer to the true nervous tissue. 



The separation of the solid nervous system from the epiblast 

 t-ikes place relatively very late ; and, before it has been completed, 

 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 epiblast at its point of we 



junction with the medullary 

 keel ; and the optic vesicles 

 spring as solid outgrowths. 

 from part of the keel itself. 

 The olfactory pits are barely 

 indicated as thickenings of the 

 nervous layer of the epiblast. 



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 Gb'tte (No. 63) to 

 speak of a special-sense plate, 

 belonging to the central nervous 

 system and not to the skin, from 

 which all the organs of special 

 sense are developed ; and to con- 

 clude that a serial homology 

 exists between these organs in 

 their development. A compari- 

 son between Teleostei and other 

 forms shews that this view can- 

 not be upheld ; even in Teleostei 

 the auditory and olfactory rudi- 

 ments arise rather from the 

 epiblast at the sides of the brain 

 than from the brain itself, while 

 the optic vesicles spring from 

 the first directly from the me- 

 dullary keel, and are therefore 

 connected with the central ner- 

 vous system rather than with the 

 external epiblast. In a slightly 

 later stage the different 



Hit,' 



B 





FlG. 32. TWO TRANSVERSE SECTIONS OF 



GNATHUS. (After Calbeiia.) 



A. Younger stage before the definite es- 

 tablishment of the notochord. 



B. Older stage. 



The epidermic layer of the epiblast is re- 

 presented in black. 



ep.. epidermic layer of epiblast ; me. neural 

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

 notochord. 

 tions of the two sets of sense 



organs is conclusively shewn by the fact that, on the separation of the 

 central nervous system from the epiblast, the optic vesicles remain attached 



