554 Stewart Paton 



large bündle forniing the ventral branch of the spinal nerves one 

 freqiiently sees tibrils coniiug from the Sympathetic, dividing to form 

 a T-shaped brauch, one arm of which nins dorsally parallel to the 

 spinal nerves, while the other is continued ventrally. An interesting 

 fact is that the size of these tilaments is the same as that of the 

 main trunk. Their dcstiuation is not known. Only in rare instan- 

 ces may filaraents be detected approaching the various orgaus which 

 are supplied by the Sympathetic. In the present paper no attempt 

 will be made to discuss in detail the various jioints connected with 

 the developnient of the Sympathetic except to refer to the Impression 

 gained by the careful study of a large uumber of seetions as not being 

 antagouistic to the view that parts of the Sympathetic may even at 

 this period be functioually active. It is however very apparent that 

 from a morphological Standpoint the development of this system has 

 proceeded more rapidly than investigators have hitherto believed to 

 be the case. 



b) Histological Studies. 



So much has been written concerning the earliest stages of the 

 development of the nervous system, that there is no occasion for 

 repeating what has already been said upon this subject. The ap- 

 l)earances of the nervous system prior to the period represented by 

 Selachian embryos of 4,5 mm. in length will therefore not be 

 described. The details of the method used in preparing the ma- 

 terial for study are given in Section 3. 



The following conditions were observed in a series of seetions 

 taken from a Pristiurus embryo of 5 mm. in length in which it had 

 been noted that the heart beat rhythmically but slowly, at the rate 

 of 19 or 20 to the minute while passive lateral movement? of the 

 head and tail, the result of the abrupt jerky contractions of the body, 

 80 eminently characteristic of this stage of the development, were 

 also present. 



In the transverse seetions of the spinal cord most of the nuclei 

 were ehmgated and oval of from lOu — 12 u in length and 3/< — Tu 

 in breadth, while a few only were circular. Some of the nuclei in 

 the inner cell layer had begun to stain deeper than others and, as 

 becomes evident later on this fact marks the beginning ditferentia- 

 tion of the si)ongioblasts. Except in the species where the large 

 cells of Reakd begin to make their appearance no other distinguishing 



