74 FORMATION OF THE LAYERS. 



all the organs of special sense are developed ; and to conclude that a serial 

 homology exists between these organs in their development. A comparison 

 between Teleostei apd other forms shews that this view cannot be upheld ; 

 even in Teleostei the auditory and olfactory rudiments 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 medullary keel, and are 

 therefore connected with the central nervous system rather than with the 

 external epiblast. In a slightly later stage the different connections 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 to the former, while the auditory and olfactory vesicles are 

 continuous with the latter. 



After its separation from the central nervous system the 

 remainder of the epiblast gives rise to the skin, etc., and most 

 probably the epidermic stratum develops into the outer layer of 

 the epidermis and the nervous stratum into the mucous layer. 

 The parts of the organs of special sense, which arise from the 

 epiblast, are developed from the nervous layer. In the Trout 

 (Oellacher, No. 72) both layers are continued over the yolk- 

 sack; but in Clupeus and Gasterosteus only the epidermic has 

 this extension. According to Gotte the distinction between the 

 two layers becomes lost in the later embryonic stages. 



Although it is thoroughly established that the mesoblast 

 originates from the lower of the two layers of the thickened 

 embryonic rim, it is nevertheless not quite certain whether it is 

 a continuous layer between the epiblast and hypoblast, or 

 whether it forms two lateral masses as in Elasmobranchs. The 

 majority of observers take the former view, while Calberla is 

 inclined to adopt the latter. In the median line of the embryo 

 underneath the medullary groove there are undoubtedly from 

 the first certain cells which eventually give rise to the notochord ; 

 and it is these cells the nature of which is in doubt. They are 

 certainly at first very indistinctly separated from the mesoblast 

 on the two sides, and Calberla also finds that there is no sharp 

 line separating them from the secondary hypoblast (fig. 32 A). 

 Whatever may be the origin of the notochord the mesoblast 

 very soon forms two lateral plates, one on each side of the body, 

 and between them is placed the notochord (fig. 32 B). The 

 general fate of the two mesoblast plates is the same as in Elas- 

 mobranchs. They are at first quite solid and exhibit relatively 



