26 STRUCTURE OF THE VERTEBRATES 



the hypomere. Eventually, both the epimere and the hypomere 

 reach the raid-ventral line, each meeting its mate which de- 

 veloped on the other side. The coelomic cavity is now almost 

 complete. Further development consists in strengthening the 

 dorsal mesentery which supports the gut; the absorption of most 

 of the ventral mesentery; and the disappearance of the septa 

 which up to this time have divided the coelomic cavity into 

 separate pockets. When complete the coelom is a continuous 

 cavity from anterior to posterior, with no separation of the two 

 sides at the bottom, but divided dorsally by the mesentery. 



Gill Slits develop in the anterior, pharyngeal part of the 

 enteron. These begin as paired outpocketings of the endoderm, 

 the bubble-like structures growing laterally until they reach the 

 ectoderm. The ectoderm pits in at these points, fuses with the 

 endoderm, and then breaks through to form the gill slit which 

 connects the pharynx with the outside. At first the slits are 

 metameric, but during development metamerism is lost in these 

 structures. 



The mouth is formed by the anterior wall of the gut coming 

 in contact with the ectoderm and breaking through. In 

 Amphioxus the mouth opening is at first asymmetrical, but it 

 later moves into its normal terminal position. The heart, or ven- 

 tral aorta, is at first a thin tube of mesodermal cells lying ventral 

 to the gut, but later it is surrounded on either side by the hypo- 

 mere. The latter develops into the cardiac muscle which gives 

 the heart its strength and rhythmic beat. 



Importance of Amphioxus. More time has been given to the 

 structure and development of Amphioxus than the student may 

 feel is necessary, but the group illustrates perfectly the funda- 

 mental plan of the entire chordate group. Its embryology is so 

 generalized that it clears many doubtful points in the develop- 

 ment of the true vertebrates, and the developmental approach 

 to comparative anatomy needs no defense. 



The importance of the group lies in the fact that in no other 

 animals are the essential chordate characteristics shown as 

 simply and with so few complications. Particularly is this true 

 of the young animal. The adult has numerous specializations, 

 and the student should review the meaning of "generalized" and 



