ioo THE AMPH1OXUS. 



In his first treatise Kowalevsky states that the 

 notochord, even during the closing of the dorsal 

 ridge, arises like a row of cells composed of an incon- 

 siderable number of large ones. He does not in his 

 second work explain these inaccurate statements, but 

 passes over them in silence. 



It may well be gathered from this second work 

 that the notochord is commenced somewhat later. 

 It was also recognised that it arises from the hypo- 

 blast, and indication was given of the importance of 

 a median dorsal hypoblast fold for the development 

 of it. 



Kowalevsky states that the hypoblast in its upper 

 half falls into three folds. He describes further that 

 in one section through an embryo with about four 

 mesoblastic somites "in the middle line is found a 

 small fold, or more exactly a channel, since the latter 

 is scarcely marked off above." In an older embryo 

 with perhaps eight mesoblastic somites (its transverse 

 sections are not correctly represented) is seen u the 

 chorda dorsalis, sharply separated from the sur- 

 rounding tissues, and under it a very fine lamina 

 of the glandular layer of the alimentary canal." 

 Respecting the number or size of the cells which 



