The Reactions of the Vertebrate Embryo to Stimulation etc. 561 



iug- the lattei' aud the narrow band (marginai veil) along the outer 

 edge of the cord. The Inter- nuclear Strands are also apparently 

 uuited along the margin of the centrai canal by a band whose vary- 

 ing width depends upon the proximity of the nuclei to the canal. 

 The matrix in ali parts seems either to be homogeneous and finely 

 granular, or in preparations fixed in sublimate -acetic and stained by 

 hsematoxylin-eosin, to be very faiutly fibrillar, giving the appearance 

 iudicated by Hensen (Die Entwicklungsmechanik etc. , Fig. 58 a 

 and 58 b) as well as other observers. At present no attempi will 

 be made to decide which one of these two structural forms represents 

 the true characteristics of the matrix. This primitive fibrillation has 

 apparently no connection with the development of the neurofibrils. 

 The very faint primary striation is observed at the point where 

 the true neurofibrils develope as well as between the nuclei 

 surrounding the centrai canal and only in very rare instances 

 has even a Suggestion of this form of structure been found in the 

 bridges. The primitive neurofibrils are coarse , deeply stained struc- 

 tures appearing primarily in a locality where more than in any other 

 place the ground substance, even after sublimate fixation, seems to 

 be granular in character, while the more delicate and attenuated 

 filaraents only become visible at later stages. (Compare Figs. 1, 2, 

 3, 7, 9 with 11, Piate 23 and with Fig. 22, Piate 25.) 



In embryos of from 5 mm. to 5,5 mm. a change has takeu place, 

 and either within the myotome or the substance of the bridges con- 

 nected with the ventral roots, eoarse deeply stained, slightly tor- 

 tuous fibrils begin to appear. These vary in length from òf^i to 

 15 /t and lie quite free in the surrounding protoplasm. In sections 

 which bave been carefuUy dififerentiated it is impossible to show 

 that they are connected with any celi. The apparent independence 

 of these primitive neurofibrils in the ventral roots from cells is one 

 of their distinguishing characteristics, but in the large cells of 

 Beard a ditferent arrangement exists. There the fibrils appear in 

 the apical process of the celi dose to its nucleus (compare Fig. 2 

 with Fig. 12, Piate 23). Unfortunately at present, one cannot speak 

 with any degree of certainty regarding the exact relation of the 

 primitive fibrils in the ventral root area to the adjacent nuclei as 

 there is no method of fixing and staining material which gives both 

 a Sharp delineation of celi boundaries and a clear picture of 

 the neurofibrils. If sections of the cord of Selachian embryos of 

 from 4 mm. — 5 mm. in length, fixed and stained by any of the me- 



36* 



