General Plan of Organization. 29 



Embryonic Plan of the Organ Systems. — In the individual organ- 

 systems the main features of the general plan, as estimated on embryonic 

 development, may be outlined as follows: 



1. The formation of an axial skeletal support, consisting primarily 

 of a strand of cellular tissue, the notochord, and secondarily of a seg- 

 mented cartilaginous, afterwards bony, vertebral column. 



2. The formation of (a) a primary cartilage skull (chondrocranium) 

 as a support for the brain and capsules of 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 muscle in a dorsolateral position 

 along the axis. 



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

 matter (neural tube), lying on the dorsal side of the axial support, and 

 differentiated into a general posterior portion, the spinal cord, and an 

 anterior expanded 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 ventral 

 wall of the digestive tube, afterwards connected with the outside of the 

 body by accessory respiratory tracts traversing the head. 



7. The formation of the circulatory system primarily on an aquatic 

 plan. This involves the formation of (a) the heart in a ventral position 

 to the digestive tube and immediately behind the gills; (b) a ventral 

 aorta, passing forward to the gills, and dividing into a paired series of 

 branchial aortic arches; (c) a dorsal aorta, combining the aortic 

 arches, and passing backward along the ventral surface of the 

 axial support; and (d) a series of paired veins returning the blood from 

 various parts of the body to the heart. 



9. The formation of the reproductive organs or gonads in association 

 with the dorsal lining of the coelomic cavity, and their connection with 

 the outside of the body by modified kidney ducts. 



10. The formation of the kidneys, either as embryonic or permanent 

 structures, from an intermediate mass of tissue, lying in general between 

 the dorsal musculature and the lining of the coelomic cavity (cf. position 

 of embryonic kidney in Fig. 20, n.). 



11. The formation, in the ventral portion of the body, of an extensive 

 space, the coelomic cavity or coelom, afterwards differentiated into 

 pericardial, pleural and peritoneal portions. 



