THE NOTOCHORD AND ALIMENTARY CANAL 443 



of any kind exist ; this smoothness is due to the presence of a very 

 well-defined cuticular layer secreted by the underlying epithelial cells. 

 This cuticle is very much thicker than is usually found in vertebrates, 

 and, strangely enough, has been thought to contain chitin. Whether 

 it really contains chitin or not I am unable to say, but it certainly 

 resembles a chitinous layer in one respect ; it is perforated by innu- 

 merable very fine tubes or canaliculi, along which, by appropriate 

 staining, it is easy to see the secretion of the underlying cell pass 

 to the exterior (Fig. 140). This marked digestive power of the skin 

 of Ammocoetes, together with the easy passage of the secretion 

 through the thin cuticular layer, renders it almost certain that a tube 

 formed from the deep ventral groove of the trilobite would, from the 

 very first, act as a digestive as well as an absorbent tube ; in other 

 words, the notochord as soon as formed was able to act as an accessory 

 digestive tube. 



This suggested origin of the notochord from a groove along the mid- 

 ventral surface of the body not only indicates a starting-point from a 

 markedly segmented portion of the body, but also points to its forma- 

 tion at a stage previous to the formation of the operculum by the 

 fusion of the two foremost mesosomatic appendages — indicates there- 

 fore its formation at a stage more nearly allied to the trilobite than to 

 the sea-scorpion. The chance of ever finding any direct evidence of 

 such a chordate trilobite stage appears to me exceedingly improbable, 

 and I greatly fear that this conception of the mode of formation of 

 the notochord can never be put to direct proof, but must always 

 remain guesswork. 



On the other hand, evidence of a kind in favour of its origin from 

 a segmented part of the body does exist, and that evidence has this 

 special value, that it is found only in that most primitive animal, 

 Amphioxus. 



This evidence is as follows : — 



At fairly regular intervals, the sheath of the notochord is inter- 

 rupted on each side of the mid-dorsal line by a series of holes, which 

 penetrate the whole thickness of the sheath. This dorsal part is 

 pressed closely against the spinal cord, and through these holes fibres 

 appear to pass from the spinal cord to the interior of the notochord. 

 So greatly do these fibres present the appearance of ventral roots to 

 the notochord, that Miss Piatt looks upon them as paired motor roots 

 to the notochord, or at all events as once having been such motor 



