CYCLOSTOMATA. 



71 



tion of fresh epiblast cells from the yolk cells (fig. 87) ; and Xill after 

 the complete enclosure of the yolk cells there is never present a sharp 

 line of demarcation between the two groups of cells. By the time 

 that the segmentation cavity is obliterated the whole yolk is enclosed 

 by the epiblast. The yolk cells adjoining the opening of the mesen- 

 teron are the latest to be covered in, and on their enclosure this 

 opening constitutes the whole of the blastopore. The epiblast is 

 composed of a single row of columnar cells. 



Mesoblast and notochord. During the above changes the meso- 

 blast becomes established. It arises, as in Elasmobranchs, in the 

 form of two plates derived from the primitive hypoblast. During the 

 invagination to form the mesenteron some of the hypoblast cells on 

 each side of the invaginated layer become smaller, and marked off as 

 two imperfect plates (fig. 38, ms). It is difficult to say whether 

 these plates are entirely derived from invaginated cells, or are in part 

 directly formed from the pre-existing yolk cells, but I am inclined to 

 adopt the latter view ; the ventral extension of the mesoblast plates 

 undoubtedly takes place at the expense of the yolk cells. The meso- 

 blast plates soon become more definite, and form (fig. 39, tns) well- 

 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 how- 

 ever become constituted of a 

 single layer only. When the 

 mesoblast is fairly established, 

 the lateral parts of the hypo- 

 blast grow inwards underneath 

 the axial part, so that the latter 

 (fig. 89, ch) first becomes isolated 

 as an axial cord, and is next in- 

 closed between the medullary 

 cord (nc) (which has by this time 

 been formed) and a continuous 

 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 features in the development of the notochord 

 were first established by Calberla^ (No. 78). 



^ In Calberla's figure, sbewinp: the development of the notochord, the limits of 

 mesoblast and hypoblast are wrongly indicated. 



Fig. 39. Tkansverse section through 



AN EMBUYO OF PETBOilYZON PlANEEI OF 208 

 HOURS. 



The figure illustrates the formation of 

 the neural cord and of the notochord. 



ms. mesoblast; n.c. neural cord; ch. noto- 

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



