EMBRYONIC PLAN OF THE SYSTEMS. 49 
2. The formation of (a) a primary cartilage skull (chondro- 
cranium) 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 expended 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 tub-, afterwards connected with the out- 
side 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. 
g. 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 coelo- 
mic cavity (cf. position of embryonic kidney in Fig. 23). 
11. The formation, in the ventral portion of the body, of an 
extensive space, the coelomic cavity or coelom, afterwards differen- 
tiated into pericardial, pleural and peritoneal portions. 
