130 THE BLASTOPORE OF THE FEOg's EGG. 



of the embryo (M, Fig. 6). A line of pigment appears in the yolk 

 before the actual separation takes place, and the slit is at first very 

 narrow, so that the two walls are almost in contact. At a later 

 stage the floor of the segmentation- cavity (S, Fig. 6) is depressed, 

 and a cavity is produced which forms part of the alimentary canal 

 of the embryo. 



The slit was formerly described as being formed by a process 

 of invagination from the epiblast, but Professor Milnes Marshall 

 has, I believe, clearly shown that this is incorrect, and that the 

 alimentary tract is formed as has now been described. Thus it 

 will be seen that the hypoblast which lines the embryonic canal is 

 derived from yolk-cells. 



The circular aperture, LB-LB' (Fig. 6), is spoken of as the 

 blastopore, and in the section under the microscope is shown the 

 commencing formation of the hy|)oblast from its dorsal lip. 



In conclusion, I desire to acknowledge my indebtedness to the 

 works of Prof. Milnes Marshall, and to thank Mr. E. Gr. Farncombe 

 for kindly preparing the drawings which illustrate this paper. 



EXPLANATION OF PLATE VIII. 

 Fig. 



1 . The egg of the frog just before the completion of the first cleft. 



2. The egg after the formation of the first equatorial cleft. N, one of the 



nuclei. 



3. A later stage in the development. S, the segmentation-cavity. 



4. The ovum at the close of segmentation. S, the segmentation-cavity. 



5. A further stage, showing the epiblast (Ep) extending round the egg. EN, 



differentiated portion of the epiblast. LB, the point at which the slit-like 

 aperture appears. S, the segmentation-cavity. Y, yolk-cells. 



6. Showing the formation of the hypoblast (H) from the yolk-cells (T). 



EN, differentiated ])ortion of the epil)last (Ep). LB, the dorsal lip of the 

 blastopore. LB', the ventral lip of the blastopore. JI, the mesenteron 

 or alimentary canal. S, the segmentation-cavity. 

 The figures are highly magnified. 



