FATE OF BLASTOPORE, ETC., IX CHELONIA. g]^ 



" It must be cle:vrlv understood tliat the orowth of the whole 

 edge of the blastoderm has so far been a uniform one. The indenta- 

 tion in the embryonic rim advances equally (after its first establish- 

 ment) wifh the more prominent parts of tlie embryonic rim called the 

 caudal swellings. * * * * 



" After a certain time the caudal swellings and the part between 

 them begin to grow more rapidly than the adjacent portions of the 

 edge of the l)las{odei-m and come to project beyond the latter like a 

 kind of tongue overhanging the yolk (Fig. 2*). * * * 



" I now wish the reader to concentrate his attention upon the 

 projecting tongue winch will form the urjder part of the embryo. * * 



" * * * — the sides of the projecting tongue become bent 

 ventral wards and towards each other until they meet or nearly meet 

 in the ventral middle line. Now two important structural results, 

 wdiich should be noted and understood follow from this l)ending : (/) 

 the two angles formed by the junction of the edge of the blastoderm 

 in the embryonic region with the edge of the blastoderm in the 

 non-eml;)ryonic region —the angles, one of which is marked a in fig. 

 2,* become closely approximated ventrally beneath the embryo ; and 

 (f/) a space is enclosed on the ventral side of the eml)ryo, wdiich space 

 is lined by endoderm, and opens ventrally to the exterior through a 

 slit formed by the contact of the ventrally bent edges of the tongue, 

 and dorsally into tlie neural canal ])y the slit in the medullary plate. 

 This space is the hind gut, and the two slits whicli are continuous 

 with one another round the hind end of the embryo are p<3rtions of 

 the blastopore. V^y the time tliat the two angles marked a and the 

 edges of the embryonic part of the blastoderm have come into contact 

 ventrally, the non-emln-yonic edge of the blastoderm adjacent to the 

 embryo have grown backwards over the yolk to form the bay 



* See the figures copied from Sedgwick on PI. VII. 



