ON THE F0R:\[ATI0X OF THE GERMINAL LAYERS TX PETROMYZOX. 139 



liead-region of an embryo n little more ndvanced. The neural cord 

 has made a considerable progress and liy its downward growth has 

 pressed down the median portion of the gut-roof, /. c. the chorda- 

 anlage. The latter is formed of a regular row of rather high columnar 

 cells. Its lateral continuation on each side is reflected upwards and 

 forms a deep diverticulum of the enteric cavit}'. This forcibly 

 reminds us of the well known section of Ampliioxm and the sequel 

 shows that the resemblance is of a deep significance. 



In fig. 21 taken from a somewhat advanced embryo, the develop- 

 ment has advanced considerably. The chorda has now been completely 

 isolated from its mother-layer, and lies firmly appressed with its upper 

 margin against the neural cord, in which it causes a slight indentation. 

 Owing to this isolation, the part of the gut-wall which was before 

 situated immediately lateral to the chorda-rt??/rt</^, has noAv slipped 

 itself below the chorda, shutting the latter fr<mi the enteric cavity. 

 Hypochordal cells Qich) are derived from this part. On each side of 

 the median line, a conspicuous ridge or fi^ld {x) of the outer wall of 

 the enteric cavity has raised and pushed itself towards the median 

 chorda-prominence, thus reducing the previously wide gut-diverticulum 

 to a mere slit and by that process separating off a triangular m.ass of 

 cells, the mesoblast (ines.f.). 



In fig. 22, which represents a still more advanced stao-e, the 

 mesoblast mass on each side has been entirely cut off from tlie wall of 

 the enteric cavity. . The constriction is solely brought about by the 

 inward growtli of the prominent ridge or fold of the outer wall of the 

 enteric ca^■ity pre\ iously referred to. Tlie inner end of the ridge strikes 

 itself against the mesol)lastic layer opposite it, surrounding the chorda. 

 It does not liowever coalesce with the same, but slips fiu'ther down- 

 wards beneath tlie chorda and tlie hypochordal cells. At the same 

 time, the two limbs of the prominent fold become separated : the 



