CYCLOSTOMATA. 



defined structures, triangular in section, on the two sides of the 

 middle line. 



At the time the mesoblast is first formed the hypoblast cells, 

 which roof the mesenteron, are often imperfectly two layers 

 thick (fig. 38). They soon 

 however become constitu- 

 ted of a single layer only. 

 When the mesoblast is fair- 

 ly established, the lateral 

 parts of the hypoblast grow 

 inwards underneath the 

 axial part, so that the latter 

 (fig. 39, c/i) first becomes 

 isolated as an axial cord, 

 and is next inclosed be- 

 tween the medullary cord 

 (nc) (which has by this time 

 been formed) and a con- 

 tinuous sheet of hypoblast 

 below (fig. 40). Here its 

 cells divide and it becomes the notochord. The notochord is 

 thus bodily formed out of the axial portion of the primitive 

 hypoblast. Its mode of origin may be compared with that in 

 Amphioxus, in which an 

 axial fold of the archenteric 

 wall is constricted off as the 

 notochord. The above fea- 

 tures in the development of 

 the notochord were first es- 

 tablished by Calberla 1 (No. 

 78). 



General history of the de- 

 velopment. Up to about the 

 time when the enclosure of 

 the hypoblast by the epiblast is completed, no external traces 

 are visible of any of the organs of the embryo ; but about this 

 time, i.e. about 180 hours after impregnation, the rudiment of 



FIG. 39. TRANSVERSE SECTION THROUGH 

 AN EMBRYO OF PETROMYZON PLANERI OF 



208 HOURS. 



The figure illustrates the formation of the 

 neural cord and of the notochord. 



ms. mesoblast ; nc. neural cord ; ch. noto- 

 chord ; yk. yolk-cells ; al. alimentary canal. 



m C. 



FIG. 40. TRANSVERSE SECTION THROUGH 

 PART OF AN EMBRYO OF PETROMYZON PLA- 

 NERI OF 256 HOURS. 



m.c. medullary cord ; ch. notochord ; al. 

 alimentary canal ; ms. mesoblastic plate. 



1 In Calberla's figure, shewing the development of the notochord, the limits of 

 mesoblast and hypoblast are wrongly indicated. 



