182 
PROFESSOR A. M. PATERSON ON THE DEVELOPMENT OP 
Again, the gangliated cords of invertebrates, which are formed from epiblast, are at 
first developed as solid, uniform, and unconstricted columns, whicb only secondarily 
become constricted and gangliated. A second reason in support of the view here 
propounded is that we possess, in the gangliated cords of the sympathetic, bilaterally 
symmetrical structures, placed along the bony axis of the trunk, in relation to the 
main vascular and alimentary tubes. On the one hand, there is no evidence of strict 
segmentation in the branches distributed peripherally from these cords; and, on the 
other hand, the parts over which they preside—the heart, blood-vessels, lungs, 
intestinal tract, &c.—are all morphologically unsegmented, like other parts and organs 
in the Mammalian trunk which are longitudinally placed. The cerebro-spiual axis, 
the notochord, heart, vascular system, alimentary canal and diverticula, ureters, &c., 
present no signs of being developed in a segmental manner. In fact, it might be 
formulated as a morphological law, to which the sympathetic cord is no exception— 
that longitudinally-placed structures, ivhether single and median, or laterally placed, 
are primarily and uniformly unsegmented.'^ 
2. From the morphological point of view, however, the most important conclusions 
derived from the investigations recorded above are undoubtedly the following:— 
a. 'Idle independent origin of the sympathetic system, and its secondary connexion 
with the cerebro-spinal system. 
b. Its development from the mesoblast. 
Both these points are of interest, more particnlarly because, on the one hand, the 
sympathetic system has previously been regarded as a specialised portion of the 
general nervous system ; and, on the other hand, there is no fact in embryology more 
firmly established than this, that whenever a nervous system is present, it is 
developed from the epiblast. 
As the foregoing observations are confined to Mammals—a highly specialised 
group of vertebrates—it may be objected that, until other and more primitive groups 
are investigated, it is unwise to draw conclusions of so general a nature. On the 
other hand, I would venture to urge the following considerations : — 
(l) In maintaining the independent and mesoblastic origin of tlie sympathetic, one 
is supported by the investigations of a considerable number of embryologists (p. 161), 
including Bemak, who studied the subject in the Chick, the main difference between 
Remak’s view^ of the development of the Avian symjiathetic system and the observa¬ 
tions made in Mammals being in regard to the primary or secondary segmentation of 
the main sympathetic cord. 
* The lougitudina.l muscles of the trunk form an apparent exccjjtion to this rule, but an exception 
which in reality may be said to prove the rule. Formed from the muscle plates, and strictly segmental 
in origin, they retain this character in their fully-formed coudhion, being, as regards their deeper layei’S, 
closely related to the various body segments, in relation to which they are formed. With regard to the 
fusion of the segmental masses in their supei'ficial parts to form elongated longitudinal muscles passing- 
over several segments (and joints), they follow the same laws as are found to govern the development 
and evolution of muscles in other parts of the body. 
