The Reactions of the Vertebrate Embryo to Stimulation etc. 565 



the outer zone or marginai veil. At a very early period commis- 

 sural bundles already connect both sides of the cord. Not infre- 

 quently single coarse fibrils, forming a very striking eleraent in the 

 field, sweep in a long curve from the triangulär area at the base of 

 the motor roots on one side across the ventral commissure and then 

 bend invvards until they bave reached the inner layer of cells. The 

 probable significance of these structures will be referred to later on. 



At the points at which neurohbrillation precedes most rapidly, 

 namely V7ithin the ventral root bridges and the triangulär area so 

 often described, the process spreads inw^ards in the direction of the 

 centrai canal. The single coarse tìbrils already described split up 

 at their distai ends into fine brush-like arraugements well shown 

 at [e] and [b] in Fig. 8, Piate 23. The multiplication of fibrils by 

 longitudinal division, a process similar to that described Ity Heiden- 

 hain, Apa'thy, Maurer and oihers, may be observed in this area. 

 The coarser and thicker bundles of neuiofibrils are characteristic of 

 these earlier stages of development. Later on the process of longi- 

 tudinal division which occurs splits these Strands up into finer fila- 

 ments. The contrast in this respect between the early and later 

 stages is well shown by coraparing Figs. 3 and 7 with 11, Piate 23. 



If the attention is directed to the process of difi"erentiation in 

 other parts of the cord, it will he seen that a few large deeply 

 stained nuclei are met with lying along the centrai canal with pro- 

 cesses extending towards the outer celi la3'ers but never showing 

 any evidence of fibrillation. These structures are imdoubtedly spongio- 

 blasts and may readily be distinguished from ali other elements. 

 The outer end of the process sometimes bifurcates forming a V- or 

 T-shaped branch, but as the faintly stained terminal tìlaments are 

 soon lost in the surrounding mass of protoplasin it is impossible 

 to speak with any certainty regarding their ultimate fate. 



Ali my observations upon the earlier stages of the development 

 of the spinai cord in Selachians seem to point, as already indicated, 

 to the existence of a dose union between the cells probably by 

 means of protoplasmic bonds, or, as Held has called them, plasmo- 

 desmata. This is the view originally promulgated by Paladino 

 (1894) and apparently confirmed by C. Desta (Riv. Sperim. Freniatria 

 Voi. 30 1904) and by R. Vakela de la Iglesia (Contribución al 

 estudio de la modula espinai, Madrid 1904). It is impossible to say, 

 whether the links between individuai cells are formed by separate 

 bridges, or whether there is a common matrix composed of material 



