442 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



influenced by a lowering of the center of gravity of the abdominal cavity or by a decrease in 

 size of the swim-bladder. 



In minute larval fishes the head is sometimes equipped with spikes, some of which 

 occasionally attain great length, as in holocentrids. Here it would seem that the spikes 

 and spicules contribute to the flotation of the organism perhaps within certain depth zones, 

 besides being of distinct advantage in imposing relatively high lower limits on the size of 

 hostile mouths capable of devouring the owner of the spikes. Doubtless various considera- 

 tions of hydrostatic balance and stability must set limits to the relations of the weight of 

 the head to that of the body, as well as to the distance of the head from the center of gravity 

 of the body. Thus in fishes with either very long or large heads the junction of the skull with 

 the backbone is braced in one way or another, as by coalescence of vertebrae or of neural 



arches. 



Why the Fish Skull is Divided into Separate Bones 



Some of the reasons why the fish skull is divided into separate bones may be as follows: 



(1) Since the skeleton is ultimately a product of the circulatory and connective tissue 

 systems, the individual bones are mere inert deposits made by growing tissues and these 

 tissues respond to the growth of the larger regions or organ-systems of which they are parts; 



(2) since throughout the members of any given taxonomic group there is a pretty constant 

 number of skull bones, each with its characteristic topographic position and contacts, it 

 follows that the number and position of osseous growth centers are predetermined by heredi- 

 tary factors, just as are the number and position of the chief arteries and veins that supply 

 these bony centers with their materials; (3) the boundaries between bones result either from 

 bending or movement of one part on another, or from the meeting of peripheral zones grow- 

 ing from different ossific centers; (4) such growth centers, the loci of greatest concentration 

 of growth and of bony material, seem often to be located at foci of greatest stress, while 

 weak contact sutures are located along zones of least stress; (5) almost every bone has 

 complex relations with surrounding parts, its own parts sharing the growth of functionally 

 different regions; (6) the cranial vault, the interorbital bridge and septum, the ethmo- 

 vomerine block and the keel-bone are, as it were, casts in osseous tissue of spaces left vacant 

 between more dominant organs such as the olfactory capsules, the eyes, the brain, the roof 

 of the mouth, the skin and the laterosensory canals; (7) similarly, the branchiocranium is 

 a complex cast in osseous tissue of spaces left between such embryonic pockets or folds as 

 the stomadseum, the gill-pouches, the opercular folds, etc. 



The osseous cast which is the neurocranium originally Included two layers named ecto- 

 steal and endosteal bones: the outer layer deposited on the under side of the lateral line 

 pockets, folds, skin areas, etc., and the inner layer deposited on either side of the notochord 

 and on or between deep-seated organs such as the brain, the cranial nerves and the main 

 sense organs. 



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