CVCLOSTOMATA. 



The segmentation is total and unequal, and closely resembles 

 that in the Frog's egg (Vol. II. p. 96). The upper pole is very 

 slightly whiter than the lower. A segmentation cavity is formed 

 very early, and is placed between the small cells of the upper 

 pole and the large cells of the lower pole. It is proportionately 

 larger than in the Frog ; and the roof eventually thins out so as 

 to be formed of a single row of small cells. At the sides of the 

 segmentation cavity there are always several rows of small cells, 



FIG. 38. TRANSVERSE SECTION THROUGH A PETROMYZON EMBRYO 160 HOURS 



AFTER IMPREGNATION. 

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



which gradually merge into the larger cells of the lower pole of 

 the egg. The segmentation is completed in about fifty hours. 



The segmentation is followed by an asymmetrical invagina- 

 tion (fig. 37) which leads to a mode of formation of the hypo- 

 blast fundamentally similar to that in the Frog. The process 

 has been in the main correctly described by M. Schultze 

 (No. 81). 



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. 38, al} in the direction of the segmenta- 

 tion cavity. In the course of the formation of the mesenteron 

 the segmentation cavity gradually becomes smaller, and is 



