70 



FORMATION OF THE LAYERS. 



lowed by an asymmetrical invagination (fig. 37) which leads to a 

 mode of formation of the hypoblast fundamentally similar to that in 

 the Frog, The process has been in the main correctly described by 

 M. Schultze (No. 8i). 



On the border between the large and small cells of the embryo, 

 at a point slightly below the segmentation cavity, a small circular 

 pit appears ; the roof of which is formed by an infolding of the 

 small cells, while the floor is formed of the large cells. This pit is 

 the commencing mesenteron. It soon grows deeper (fig. 37, al) and 

 extends as a well-defined tube (shewn in transverse section in fig. 88, 

 al) in the direction of the segmentation cavity. In the course of the 



Fig. 38. Tuansvkusk section thkough a Petromyzox embryo 160 hours after 



impregnation. 

 ep. epiblast; al. mesenterou; yk. yolk-cells; ms. mesoblast. 



formation of the mesenteron the segmentation cavity gradually 

 becomes smaller, and is finally (about the 200th hour) obliterated. 

 The roof of the mesenteron is formed by the continued invagination 

 of small cells, and its floor is composed of large yolk-cells. The wide 

 external opening is arched over dorsally by a somewhat prominent 

 lip — the homologue of the embryonic rim. The opening persists 

 \ till nearly the time of hatching ; but eventually becomes closed, 

 I and is not converted into the permanent anus. On the f )rmation of 

 'the mesenteron the hypoblast is composed of two groups of cells, 

 (1) the yolk-cells, and (2) the cells forming the roof of the mesen- 

 teron. 



While the above changes are taking place, the small cells, or as 

 they may now be called the epiblast cells, gradually spread over the 

 large yolk-cells, as in normal types of epibolic invagination. The 

 growth over the yolk-cells is not symmetrical, but is most rapid in 

 the meridian opposite the opening of the alimentary cavity, so that 

 the latter is left in a bay (cf. Elasmobranchii, p. 51). The epibolic 

 invagination takes place as in Molluscs and many other forms, not 

 eimply by the division of pre-existing epiblast cells, but by the forma- 



