VII.] THE EPIBLAST. 197 



organs are simply marked out by greater or less condensation 

 of the simple mesoblastic substance. 



From the fifth day onwards, however, histological differ- 

 entiation takes place rapidly ; and it soon becomes possible 

 to speak of this or that part as being composed of muscular, 

 or cartilaginous, or connective &c. tissue. It is not within 

 the scope of the present work to treat in detail of these histo- 

 genetic changes, for information concerning which we would 

 refer the reader to histological treatises. We have already 

 had occasion to refer 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 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 caudate nerve-cells. There is, however, some 

 doubt whether mesoblast cells may not possibly enter into 

 its formation, and it is very probable that the white matter 

 of the brain and spinal cord is derived from the mesoblast 

 alone. 



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 epidermis ; 

 not however the dermis, which is of mesoblastic origin. 

 The line of junction between the epiblast and the meso- 

 blast coincides with that between the epidermis and the 

 dermis. From the epiblast are formed all such tegumentary 

 organs or parts of organs as are epidermic in nature. 



In addition to these, the epiblast plays an important 

 part in the formation of the organs of special sense. 



According to their mode of formation, these organs 

 may be arranged into two divisions. In the first come the 

 eases where the sensory expansion of the organ of special 



