THE SYMPATHETIC NERVOUS SYSTEM IN MAMMALS. 
161 
embryologlcai considerations, that the superior cervical ganglion is to be looked upon 
neither morphologically nor physiologically as a fundamental portion of the gangliated 
cord, but rather as a distal or collateral ganglion, makes it still more difficult to regard 
the gangliation of the chain as indicative of its metameric constitution. 
It is noteworthy that the central connexions of the main chain form the only 
evidence of segmentation, which is not discredited by exceptional characters. These 
connexions are obviously truly segmental, being intimately related to the spinal 
nerves; and the segmental hypothesis of the formation of the sympathetic is founded 
essentially on these anatomical relations. Hereafter I shall endeavour to demon¬ 
strate that the sympathetic is primarily unsegmented, and shall attempt to show 
adequate cause for the appearance of segmentation in the cord. 
C. Emhrijologij. 
The recent memoirs bearing on the development of the sympathetic are com¬ 
paratively few, and the excellent account given of the literature of the subject in 
Onodi’s monograph (15), makes it unnecessary to repeat it here at length. Various 
steps in the process have been traced by previous observers; but there is absolute 
divergence in their views regarding the most vital points. In the first place it cannot 
be said to be definitely proved from which of the embryonic layers the gangliated cord 
arises. On the one hand we have the weighty authority and historical researches 
of Kemak (5, 6), supported by Bidder and Kupffer (7), Kolliker (8), ITensen (9), 
His 'I (10), Gotte (11), and Schwalbe (12), in defence of its origin from mesoblast. 
On the other hand Balfour’s work on the development of Elasmobranchs (13), 
the memoir of Schenck and Birdsell (14), and the elaborate monograph of Onodi 
support the idea of its origin from epiblast. There is a third hypothesis to the effect 
that the sympathetic is originally derived from hypoblast. As far as I am aware, 
this theory has never been seriously supported by any observer, and, as it will not 
bear investigation, it is not necessary to discuss it further. 
Those who, like Bemak and his followers, uphold the mesodermal origin of the 
sympathetic, maintain that it springs from isolated segmental ganglia arising from 
the mesoblast in the neighbourhood of the protovertebrse, which ganglia obtain con¬ 
nexions with the periphery, with the spinal nerves, and aiitero-posteriorly with the 
contiguous ganglia. 
Balfour (13) was the first to suggest the ectodermal origin of the sympathetic, in 
order to harmonise it with the rest of the nervous system. In his monograph on the 
development of Elasmobranchs, he makes some observations on the development of 
the ganglia. He says : “ There may be seen short branches from the spinal nerves, 
which take a course towards the median line of the body, and terminate in small 
irregular cellular masses immediately dorsal to the cardinal veins. These form the 
first traces that have come under my notice of the sympathetic nervous system. In 
MDCCCXC.—B. Y 
