ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
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the sides of the body of the embryo form the rudiments of the musculature 
of the body. 
Each of these cells gives rise to a primitive fibre, by withdrawing its 
pseudopodial expansions, and growing by the addition of new proto- 
plasmic material ; this latter is not formed of granular plasm, like that 
already existing, but of contractile substance. This substance is at first 
deposited at the two extremities of the cell, then extends over the entire 
periphery, the deposit being always most abundant towards the ends ; 
in this way the element grows along its longitudinal axis. The primitive 
granular protoplasm with the nucleus which it incloses thus becomes 
enveloped and set in the middle of a sheath of contractile substance. 
The differentiation into fibrils commences towards the centre and not the 
periphery of the cell ; a transverse section taken at the level of the 
nucleus shows this nucleus in the very axis of the fibre, and from within 
outwards, the granular protoplasm, the deep contractile substance divided 
into fibrils, and the still homogeneous contractile peripheral substance. 
This is different to what obtains in Vertebrates; the contractile 
substance in them appears first on one or both surfaces of the primitive 
element, and not at its two extremities ; moreover, the primitive fibrils 
first appear towards the periphery of the cell, and not in its central 
region. The differences may be explained as due to the epithelial 
origin of the somatic muscular fibres of Vertebrates, and confirm, while 
they extend, the views of the brothers Hertwig as to the nature of the 
coelom. 
Both the epithelial as well as the mesenchymatous types of origin 
apply to smooth as much as to striated fibres; the smooth fibres of 
Nematodes, for example, are developed in the same way as the striated 
somatic fibres of Vertebrates, and the smooth muscles of Molluscs like 
the striated fibres of Arthropods. In both cases the nucleus is occasion- 
ally single, and this is frequently the case with smooth fibres, while it 
sometimes multiplies and converts the primitive element into a multi- 
nucleated primitive fibre. 
Development of Sympathetic Nervous System in Mammals.*— 
Prof. A. M. Paterson has investigated the development of the main sympa- 
thetic system, chiefly in Bodents. The first event is the development of 
the main sympathetic cord. It is formed as a cellular rod or column, 
uniform in outline, and without ganglia or constrictions. It appears in 
and is derived from the mesoblastic tissue on either side of the embry- 
onic aorta, and in front of the growing vertebral column. The cord appears 
after the formation of the roots and ganglia of the spinal nerves, and is 
at first entirely independent of them. 
The connection with the spinal nerves is secondary ; the inferior 
primary division of a (typical) spinal nerve divides, on reaching the 
junction of body-wall and splanchnopleure, into a somatic and a splanch- 
nic branch. The latter gradually grows mesially and ventrally, and 
finally becomes connected with the sympathetic ; some part of it does not 
join, but passes on into the splanchnic area. In the anterior part, 
however, of the thorax the whole of the splanchnic branch appears to be 
joined to the sympathetic cord. 
* Phil. Trans., 181 B. (1891) pp. 159-86 (9 pis.). 
