186 BULLETIN OF THE 



ing concavities in the spinal cord. The nerve fibres primarily connect- 

 ing the spinal ganglia with the cord, form an almost continuous sheet 

 extending along each side of the spinal cord. At intervals correspond- 

 ing to the interspaces between the protovertebrse, these fibres are appar- 

 ently pushed to either side by the thickened mesoderm which projects 

 upward from the region of the notochord, so that the fibres between 

 every two such mesodermic thickenings are drawn into one spinal 

 ganglion. The width, therefore, of the band of fibres connecting a 

 spinal ganglion with the spinal cord, corresponds to the width of the 

 protovertebra lateral to it (Figs. 13, 13 a). From a glance at the gen- 

 ei - al relation of the neural concavities to the nerves which arise from 

 them, it will be noticed that, where the ganglion is connected with the 

 neural axis by a mass of fibres bound closely together, the curve of the 

 concavity from which they arise is sharp, as in the region of the medulla. 

 Where the fibres are spread out, forming a wide ganglionic connection, 

 the curve of the concavity is gentle, as in the spinal cord. But whether 

 its curve be gentle or sharp, the concavity in both medulla and spinal 

 cord is the source from which the nerves originate, and the neural 

 swellings which correspond to the nerve roots are the inner ridges, not 

 the outer neuromeres. 



Beraneck is unwilling to homologize the neuromeres of the medulla 

 with the swellings of the spinal cord, because of the difference in the 

 later differentiation of the nervous tissue in these two regions, and be- 

 cause, as he affirms, the swellings of the spinal cord are transient. But 

 so are also the neuromeres of the medulla. By the fifth day of incu- 

 bation they are fast fading away, yet at this time the wave-like form of 

 the walls of the spinal cord is still plainly visible (Fig. 13). Here, as in 

 the medulla, the segmentation is more manifest in the ventral region 

 than in the dorsal. Moreover, at the time when the neuromeres of the 

 medulla and the swellings of the spinal cord first appear, the tissue 

 thi'oughout the neural tract is quite indifferent. Further, since the 

 first four folds posterior to Beraneck's five medullary folds lie also in 

 the region of the medulla, and are also connected with a cranial nerve, 

 — viz. the tenth, — it is practically impossible to draw the line sepa- 

 rating these folds from the more anterior. It is true that they are 

 smaller, but the difference between the fifth neuromere and the next 

 posterior fold is not as great as the difference between the second and 

 third neuromeres. I therefore see no reason why the successive swell- 

 ings which originally mark the neural tract should not be regarded as 

 homologous structures. 



