102 THE AMPHIOXUS. 



of the mesoblastic somites, which at first does not 

 reach as far as the anterior end of the body, but only 

 as far as the anterior end of the first mesoblastic 

 somite, and gradually extends aiiteriorally. Towards 

 the posterior, too, the formation of the notochord 

 advances during the further separation of the meso- 

 blastic somites. The study of the living object yields 

 thus in these stages but few results. In later stages, 

 during the histological differentiation of the noto- 

 chord, the study of the living object is again, however, 

 of special importance in order to follow the appearance 

 of the vacuoles in the notochord. 



AVe make a far greater advance through the study 

 of prepared objects (preparations made with osmium- 

 carmin-glycerine). It can there be recognised that 

 the notochord arises from the upper wall of the 

 alimentary canal. For in the first stages, where the 

 notochord foundation can be noticed, it still takes 

 a direct share in forming the wall of the archenteron 

 (Fig. 48). It may also be seen that the founda- 

 tion which appears with several layers in the profile 

 passes over into the simple hypoblast epithelium 

 behind the mesoblastic somite region. 



In the later stages, with nine mesoblastic somites the 



