GASTRULATION 633 



hinder end of the animal, and the remnant of the exposure of 

 the white cells — the yolk plug — comes, before it disappears 

 (in the way described below), to lie in the dorsal region of that 

 end. By the same rotation the region formed by the growth of 

 the dorsal lip moves up into the position of the dorsal side of 

 the animal. The lip continues to grow over the yolk plug, thus 

 narrowing the blastopore. The narrowing, however, takes place 

 not by equally rapid ingrowth of the lip all round, but by a faster 

 growing together of the sides of the circle in its hinder part. 

 At last the plug is covered and the blastopore is a mere slit. Then 

 the middle part of this slit closes completely, its sides coming 

 together, but leaving at its ends two small openings. Of these 



the upper remains open, the last 



,g- vestige of the blastopore, and 



^I^ becomes later the neurenteric 



canal (p. 636), while the lower, 



_^, ^ though it closes, leaves in its 



*"^ '^"-ff- place a pit of ectoderm — the 



proctodaeum — in which the anus 

 an.^^lf ^^ eventually breaks through. 



Where the sides of the slit meet, 



Fig. 49^. — An embryo of the frog at , , , , . 



a later stage. between anus and neurenteric 



an., Proctodaeum (invagination which wiU form Caual, thcrC rcmamS 3. Seam 



anus) ; W/>., last vestige of blastopore ; g./>., • xi^ form of a PTOOVP the 



gill plate; n.f., neural fold; n.g., neural 1" ^UC lOrm Oi d glUUVC Llic 



groove ; pr.g.. primitive groove ; s.p., sense primitive grOOVC UUder which 



lies a band of cells — the primi- 

 tive streak — in which ectoderm, endoderm and mesoderm meet 

 and fuse. During this process an inward movement of the yolk 

 cells, which is also invagination, has obhterated the blastoccele 

 and enlarged the enteron, which was at first a mere slit, so 

 that it becomes a spacious cavity, which communicates with 

 the exterior by a slit between the dorsal semicircle of the blasto- 

 pore lip and the yolk plug. It is this change of site of the principal 

 cavity in the embryo that, by shifting the centre of gravity, 

 causes the rotation mentioned above. 



FORMATION OF MESOBLAST 



At the end of gastrulation the archenteron is a large cavity 

 whose very thick ventral wall has arisen from the yolk cells, 

 while the thinner dorsal wall has been formed by the growth 



M.Z. — 21 



