256 



K. MITSUKURI. 



and having a comparatively insignificant morpholo- 

 gical value. Although the whole yolk-sac should he regnrded as a 

 diverticulum of the archenteron and although it has a definite morpho- 

 logical value, it is a matter (^f comparative indifférence, so far as 

 morphology is concerned, whether its inside is filled with cells 

 charged with yolk-granules, or with free yolk-spheres, or with a 

 nutritive liquid or with a mixture of all three. Looked at in this 

 light, the chelonian egg is nothing hut the ani])hibian egg, with an 

 enormous ventral saccular appendage surcharged with nutritive 

 matter.* The whole yolk-sac (Woodcut I. J-X—B) must not, 

 however, he looked on as strictly homologous with the part of the 

 amphibian egg ventral to the line Z~ Y. For, in the latter, the 

 epiblast of that part becomes the ventral abdominal wall of 

 the future animal, while in Chelonia the epiblast of the yolk-bag 

 becomes later n part of the serous envelope, — the ventral a b d o m i n a 1 

 wall of the embryo being formed within the embryonic 

 shield above the yolk-sac, and the yolk-sac with the enclosing sheet 

 of hypoblast and mesoblast cells migrating within the body of the 

 embryo. When it has done so, nobody has any difficulty 

 in accepting it as an appendage of the alimentary 

 canal which has for its function the storage of nutri- 

 tive matter. My contention is that as such it should be 

 looked on from the first. The archenteron is at first so utterly 

 insignificant in size compared with the yolk-sac that the true na- 

 ture of the latter is obscured : none the less the yolk sac is a mere 

 appendage of the archenteron. This < view makes it necessary to regard 

 the primitive lower layer enclosing the yolk-sac as a part of the 



* It will 1)6 seeu that f urtlier considei-ation has made me modify in some details my views 

 as set forth in the preliminary notice sent to the Aiuitomhcher Anzeiger, and pulilished in that 

 journal, Nos. 12 & 13, 1893.. 



