VIII.] THE EPIBLAST. 271 



incidentally to many of the earliest histological events, 

 and shall content ourselves by giving a brief summary 

 of the derivation of the tissues of the adult animal from 

 the three primary layers of the blastoderm. 



The epiblast or upper layer of many embryologists 

 forms primarily two very important parts of the body, 

 viz. the central nervous system and the epidermis. 



It is from the involuted epiblast of the neural tube 

 that the whole of the grey and white matter of the 

 brain and spinal cord appears to be developed, the 

 simple columnar cells of the epiblast being apparently 

 directly transformed into the characteristic multipolar 

 nerve-cells. The whole of the sympathetic^ nervous 

 system and the peripheral nervous elements of the 

 body, including both the spinal and cranial nerves and 

 ganglia, are epiblastic in origin. 



The epithelium (ciliated in the young animal) lining 

 the canalis centralis of the spinal cord, together with 

 that lining the ventricles of the brain, all which cavities 

 and canals are, as we have seen, derivatives of the 

 primary neural canal, is the undifferentiated remnant of 

 the primitive epiblast. 



The epiblast, as we have said, also forms the epider- 

 mis, not however the dermis, which is of mesoblastic 

 origin. The line of junction between the epiblast and 

 the mesoblast coincides with that between the epidermis 



1 The details of the development of the sympathetic system have 

 only been imperfectly worked out in the chick. We propose deferring 

 our account of what is known on this head to the second part of this 

 work dealing with the MammaUa. We may here state, however, that the 

 whole of the chain of the sympathetic ganglia is developed in con- 

 tinuity with the outgrowths from the wall of the neural tube which 

 give rise to the spinal nerves. 



