18 AKT. 11. — S. HATTA. 



as in a younger ovum, as will be seen by a comparison of this 

 part with the corresjDonding part in the ovum last described {Fig. 

 13), but it constitutes a typical columnar epithelium which stands 

 in direct connection with the indifferent mass of yolk-cells. While 

 the micromeric layer is in this way undergoing histological 

 differentiation, the macromeric part also does not remain unchanged ; 

 it is produced by cell-multiplication outwards into a rounded 

 elevation (c. em.), which represents the forerunner of what I have 

 called above the conical eminence. 



The origin of the boundary groove has already been explained 

 and is not difficult to understand. Notwithstanding the eminence 

 protruded, the junction line, i. e., the connection of the 

 epithelium with the solid mass of yolk-cells, retains its original 

 position, while the thin-walled animal hemisphere above this line 

 is kept distended ])y pressure of liquid contents within it. Con- 

 sequently, a constriction or a groove is necessarily brought about 

 along the junction line as a passive result ; this is the boundary 

 groove {Mg. 14, b. g.) 



The boundary groove is very striking in the median sagittal 

 section through a little further advanced ovum {Fig. 15, h. g.), 

 in which a dorsal small part (*%;.) of the vegetative hemisphere 

 is flattened. In this section it is very well seen that the groove 

 looks striking, not because it had sunk deeper, but merely because 

 of the further protrusion of the eminence (c.em.), which now shows 

 a conical configuration when observed as an opaque object (see Figs. 

 Aa and Ab). In sections of the same stage {Fig. 15) it is seen that 

 the cells composing the eminence and the flattened part are much 

 smaller and more thickly crowded than those of the remaining 

 part of the macromeric hemisphere (this point is unfortunately 

 not well shown in the figure). Further, a part of the macromeric 



