The Reactious of the Vcrti^bratc Embryo to Stimulation etc. 551 



Passing to the stage represented by a Torpedo eiubryo of 

 14 mm. in lengtli very great changes ave uoted. Within the brain 

 and medulla the nimiber of fibrils has very greatly increased. As 

 a rule deeply stained bundles skirt the outer edge of the sections 

 through the centrai nervous system, forining long commissural traets, 

 not only between the two halves of the medulla, but between the 

 cranial nerves of the opposite side. Only in exceptionable cases do 

 fibrils seem to bring the cells in the deeper layers or along the 

 centrai canal into connection Avith those in the outer layers. That 

 the difterentiation into fibrils takes place much more vapidly in the 

 external zones of the centrai nervous system thau it does towards 

 the median line is a very striking fact. In the more frontal 

 portions of the centrai nervous system the differentiation has scar- 

 celv begun. A few scattered neurofibrils in the outer zone are 

 practically the only evidences that indicate the beginning of this 

 process; a condition in marked contrast to the progress already 

 Iliade in the medulla. 



When we study the individuai cranial nerves we are at once 

 struck by the rapidity with which the process of differentiation has 

 progressed. The Ophthalmicus profundus, ending free in the meso- 

 chyme, in the sectious in front of the piane of the eye Stands out very 

 sharply as it contaius uumerous bundles of deeply stained fibrils 

 which may he traeed with ease, as the nerve arches back of the 

 eye, into the mesocephalic gangliou (Ciliary ganglion). This latter 

 structure is a very prominent feature aud although it contains 

 many neurofibrils and is in dose contact for several sections with 

 the so-called Ciliary field no fibrils are seen to enter this epithelial 

 thickening. I also failed to detect any fibrils in the Ramus 

 dorsalis ganglii mesocephalici. Although it is an easy matter 

 to make out that undifferentiated Strands of protoplasm connect the 

 Ciliary field and ganglion I bave not been able to discover that any 

 differentiation into neurofibrils takes place in this tract in Torpedo. 

 Dr. R. Gast has called my attention to the fact that in sections of 

 Mustelus stained by the same method this connecting tract between the 

 Ciliary ganglion and periphery is well diöerentiated into neurofibrils 

 even at the early stage represented by embryos of 7,5mm. in length. 

 In the piane just in front of the point vvhere the Ganglion trigemini 

 Comes into the field a bündle of deeply stained fibrils becomes vi- 

 sible in the epidermal thickening in the region supplied by the 

 Superficial Ophthalmie, and is an important object in the area through 



