INFLUENCE OF LATERAL-LINE CANALS Ba) 
then as now. ‘The position of the canals in the ancient larvae of 
the early amphibians was probably constant and since the canals 
mechanically determined the formation of bone the relation of 
the adult elements is constant. 
This, of course, explains only the general features of the asso- 
ciation, and that is all we are seeking to explain. Why certain 
isolated segments of bone, as laid down around the larval canals, 
later unite to form single skull elements is a matter for which 
we have no uniform explanation. Since the elements of ancient 
vertebrate skulls differ in their form and arrangements only in 
details, we must assume that a variety of modifying agencies 
enter in. The form of the head and hereditary influences are 
very potent. 
Our application of embryological data to paleontology then 
consists in stating that since in modern fishes the lateral-line 
canals develop early and exert a mechanical influence on the 
formation of the peripheral osseous elements, there is no reason 
to doubt that the ancient amphibians of the Coal Measures, 
Permian and Triassic possessed in their larval stages similar 
conditions of an early developed lateral-line system which for the 
reasons stated exerted a mechanical influence on the deposition 
of the bones. 
SUMMARY 
1. The lateral-line sense organs are not trophic for the head 
skeleton. 
2. The association of lateral-line canals with certain definite 
peripheral osseous elements has been a constant feature in the 
organization of fishes and Amphibia from Devonian times to the 
present. 
3. The presence of dense connective tissue in the lateral-line 
canals of the body and of the head is important because it thus 
furnishes an inactive base for bone deposition. 
4. There are two sets of bones in the head of Amiurus which 
are here interpreted as developing in two ways because of: a) stress 
the more powerful influence and, b) the mechanical influence of 
the lateral-line canals. 
