297 
1888 .] Prof. Wilhelm His on Animal Morphology. 
curving of the body does not occur, the heart remains in its primary 
position, and no neck is formed. 
These examples, which could easily he multiplied, may he suffi- 
cient to prove the general importance of elementary mechanical 
considerations in treating morphological questions. They show at 
the same time how the means that nature uses in forming her 
organisms may be very simple. The segmented germ divides itself 
into the primitive embryonic organs by a few systems of foldings. 
The most important displacements of these primitive organs are 
the consequences of some inflections of the longitudinal axis; and 
even the most complicated of all our organic systems, the nervous 
system, follows a course of the most astonishing simplicity. 
As I communicated at the meeting of the British Association for 
Advancement of Science at Manchester (Section D, Sub-section 
Physiology), every nervous fibre issues as an outgrowth from a 
single cell, the motor fibres coming from the cells of the medullary 
tube, the sensitive fibres from those of the ganglia. The fibres 
unite into bundles and into trunks, the first trunks being very short 
and elongating slowly. They usually follow the direction in which 
they grow out, and, where there is no obstacle, they run a long way 
in a straight course. By secondary displacements the nerve-trunks 
may be curved, and so the direction of their actual ends and of their 
growth may be altered. Different nerves, growing out in crossed 
directions, may unite and form anastomoses. When outgrowing 
nerves find obstacles in their way, they undergo deviations, and as 
these are not the same for all fibres of a trunk, division will be the 
consequence. Cartilages and blood-vessels are the most frequent 
causes of this deviation and division of nerve trunks. 
Prom these statements it may perhaps seem that interferences of a 
purely accidental kind govern the disposition of the nervous ramifi- 
cations. But, as we know, the system which results from all these 
complicated events is finally seen to be of the finest organisation; 
every one of its numerous arrangements is in fixed relation to some 
functional activity, and the whole system depends in a most deli- 
cate manner upon the all-embracing law of heredity. In organic 
development there is no accidental cause ; every single process occu- 
pies its own peculiar place, and all together follow the order of the 
general periodic function of life. 
