138 THE AMPHIOXUS. 



gill, the course of the blood-vessel foundation ex- 

 periences a deviation. The clear canal is pushed to 

 the right through the foundation of the gill, it runs 

 the length of the latter 's outer edge on the right 

 side of the body, and ends off in the region of the 

 club-shaped gland (Fig. 61 A). Contractions of this 

 blood vessel are not to be observed till somewhat 

 later, after the mouth opening and first gill slit have 

 already perforated. Kowalevsky described this blood 

 vessel in somewhat older stages. When these are 

 reached, we will examine his account more closely. 

 With regard to the origin of the vessels, he conjectured 

 that they " originate from cells which lie freely in 

 the cavity of the body, which at first collect in a firm 

 strand, the lumen being only a secondary formation." 



FURTHER FORMATION AND HISTOLOGICAL DIFFERENTI- 

 ATION OF THE NOTOCHORD. 



Kowalevsky's account of the histological alterations 

 of the notochord must be regarded as incorrect. 



He speaks of a special notochord sheath ; this is 

 however not to be seen ; perhaps it was the dorsal 

 and ventral cell series of the notochord which led 

 him to make this mistake. 



