!5o 



CHORDATE ANATOMY 



SOMITES S0M3 

 MIDBRAIN 



SOMITE 7 

 I rOREGUT 



SOMITE A \ » 



SOMITE I HEART 



PERICARDIAL CAVITY 



POST -ANAL GUT 

 NEURENTERIC CANAL' 



Fig. 232. — A 7 mm. Squalus embryo viewed as a cleared specimen from the left 

 side. The yolk-sac has been mostly removed. Two gill-slits are open. Cranial nerve 

 anlagen are indicated by Roman numerals. 



ANTERIOR CAVITY 



MYOTOME 2 



CHORDA 



ENTERON 



MOUT 



ENDOSTYLE / LEFT 1ST (TRANSIENT)GILL-POUCH 



CLUB-SHAPED GLAND 



A. EARLIER LARVA. 



ENDOSTYLE 



RIGHTIST PERMANENT GILL-SLIT 



PREORAL PIT 



CLUB-SHAPED GLAND' 

 TRANSIENT GILL-SLIT 



LE FT 1ST PERMANENT GILL-SLIT 

 B. LATER LARVA. MEDIAN VENTRAL BLOODVESSEL 



Fig. 233. — A, yottng Amphioxus larva viewed from the left side as a translucent 

 object. (Redrawn after Hatschek.) 5, later larva. (Redrawn after van Wijhe.) The 

 mouth of Amphioxus becomes enormously enlarged and, by its growth backward on the 

 left side, interferes with the symmetry of development of the gill-slits. The gill-slits 

 of the left side of the body develop before those of the right side. The median line of 

 the ventral side of the pharynx is indicated by the median ventral blood-vessel. Modifi- 

 cation of function and degeneration affect the anterior three pairs of gill-slits. The 

 first pair become the endostyle. The second pair form the transient larval club- 

 shaped gland. The left third slit has no mate and soon disappears. The fourth pair of 

 slits form the first permanent pair. 



