EMBRYONIC PLAN OF THE SYSTEMS 43 



tenth cranial nerv^es and the chief arterial vessels of the heart, 

 which are more fully referred to below. 



The fundamental significance of branchiomerism lies in the fact 

 that respiration by means of gill perforations of the pharynx is 

 characteristic of that branch of the animal kingdom designated 

 Chordata. In the various invertebrate phyla are found respiratory 

 surfaces of many kinds, such as thin surface membranes, external 

 tufted, or invaginated tubes, and analogous structures, but these 

 provide for diffusion without perforation of the body tube. 



Embryoxic Plan of the Systems 



Governed by the broad principles of organization just indicated, 

 the individual organ-systems are disposed according to a general 

 plan the main features of which may be outlined as follows: 



1. The formation of an axial skeletal support, consisting 

 primarily of a strand of cellular tissue, the notochord, and secon- 

 darily of a segmented cartilaginous, afterwards bony, vertebral 

 column. 



2. The formation at the anterior end of this axial support of 

 (a) a primary cartilage skull (chondrocraniumj as a support for 

 the brain, with capsules for the special sense organs (neurocranium 

 or cerebral cranium); and (b) a series of cartilaginous visceral 

 arches (splanchnocranium or visceral cranium). 



3. The formation of the chief skeletal rnuscle in a dorsolateral 

 position along the axis. 



4. The formation of the central nervous system as a tube of 

 nerv^ous matter (neural tubej, lying on the dorsal side of the axial 

 support, and differentiated into a generalized posterior portion, the 

 spinal cord, and an expanded and specialized anterior portion, the 

 brain. 



5. The formation of the digestive tube as a median structure, 

 lying directly beneath the axial support, and of special glandular 

 appendages arising from the epithelium of its wall. 



6. The formation of the lungs as paired outgrowths of the ven- 

 tral wall of the digestive tube, afterAvards connected with the out- 

 side of the body by accessory respiratory tracts traversing the head. 



