84 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



occlusion of part of the lumen takes place in the great majority of 

 the lower Vertebrates. The side walls of the tube approach one 

 another so as to convert the rounded lumen into a vertical slit and 

 finally they come into contact and fuse so as completely to obliterate 

 the cavity except in its ventral portion which remains open as the 

 definitive central canal. 



In the case of the Bird — in which the process has been worked 

 out in detail (see Ramon y Cajal, 1909) the increase in thickness of 

 the wall of the neural tube is due primarily to the cells composing it 

 taking on a tall columnar form — the individual cell extending right 

 from the central canal to the outer surface. The cell-body becomes 

 very attenuated, with a marked dilatation containing the nucleus. 

 The nuclei become necessarily situated at different levels and this in 

 an ordinary transverse section obscures the fact that the wall is still 

 composed only of a single layer of cells. 



With subsequent development the cells become differentiated into 

 those which are actually nervous and those which remain relatively 

 indifferent and fulfil a mainly supporting function. The latter con- 

 tinue for a considerable period to traverse the whole thickness of the 

 wall. They increase greatly in length : their form becomes more 

 and more attenuated the greater part of their length becoming prac- 

 tically filamentous with small irregular projections and varicosities, 

 while the portion nearer the central canal, in the course of which the 

 nucleus is embedded, remains somewhat stouter. 



The presence of such supporting cells traversing the whole thick- 

 ness of the wall is only temporary : in later stages they are replaced 

 by the greatly branched neuroglia cells. While many authors have 

 taken the view that these latter are to be regarded as immigrant 

 mesenchyme cells — a view that has weighty general considerations 

 in its favour — Ramon y Cajal and others have adduced strong evi- 

 dence to show that they are simply the original supporting cells 

 which have withdrawn, or lost, their internal and external portions 

 and assumed a complicated branched form. 



In addition to the comparatively indifferent supporting cells 

 which have just been mentioned there are present in the wall of the 

 neural tube the numerous elements which are destined to become 

 actual neurones or nerve -cells. Such embryonic nerve -cells have 

 been termed by His neuroblasts in contradistinction to the non- 

 nervous elements or spongioblasts. 



At first isodiametric these cells like their neighbours take on a 

 tall columnar shape stretching throughout the thickness of the wall : 

 their terminal portions become more and more attenuated and they 

 present a spindle -like (bipolar) appearance. Later their shape 

 becomes pearlike the stalk being prolonged into a nerve fibre 

 (neurite, axon) while finally the development of branched projec- 

 tions (dendrites) brings about the definitive multipolar condition. 



These developing neurones lie in the spaces between the in- 

 different cells and from an early stage (third day in the case of the 



