DEVELOPMENT OF NEKVE-TISSUE AND EPITHELIUM. 1 03 



§ 81. As regards the nervous system, it has been generally 

 believed hitherto that the growth of nerve-fibres was everywhere 

 dependent on the pre-formation of embryonic cells. That the 

 repair of divided nerves took place by the intercalation of em- 

 bryonic tissue between the cut ends seemed finally agreed upon ; 

 indeed, it has not as yet been superseded by any fresh doctrine. 

 On the other hand, a tendency has shown itself of late to refer 

 the penetration of the nerve-ends into the growing parts of the 

 body, and their consequent elongation, to an apical growth 

 (Spitzenwachsthum) without any apposition of new elements. 

 As regards the brain, Besser skilfully defends the position, that 

 all the ganglion-cells of later growth exist pre-formed as so- 

 called neuroglia-nuclei, in the very earliest stages of the develop- 

 ment of this organ. 



§ 82. One important question yet remains to be answered. 

 How do the EPITHELIAL STRUCTURES groAv and renew their losses ? 

 Unfortunately, no precise answer to this query can at the present 

 time be given. We are justified, however, in provisionally 

 adopting the following conclusions. A clear line must in the 

 first j^lace be drawn between those epithelia which grow out- 

 loards and those wdiich grow inwards. The former invest the 

 entire free surface of the organism, the skin and mucous mem- 

 branes, forming an uninterrupted whole ; the latter occupy cer- 

 tain cavities dug out of the parenchyma of the body, and are 

 known as gland-cells or glandular epithelia. Both systems ori- 

 ginate in the same layer of the blastoderm ; throughout life they 

 maintain this structural continuity, in such wise that the glandu- 

 lar epithelium may be regarded as a direct prolongation and in- 

 flexion of the surface-epithelium. Yet those who w^ould com- 

 prehend the alterations of the tissues in disease must lay especial 

 stress upon the fact that the antithesis in the direction of their 

 growth alluded to above, exists from the very first, and that 

 upon it is based the essential difference between the two systems. 

 For the gland originates by the development of cellular protru- 

 sions from that side of the epithelial lamina of the blastoderm 

 which is turned towards the conjoint blood-vascular and connec- 

 tive-tissue system : these protrusions divide and subdivide as they 

 increase in length, and finally become hollow to a certain depth 

 from the free surface. Their growth is decidedly central, and 

 takes place by fission of the existing epithelial cells, which goes 



