GASTRULATION 



623 



which is as yet very wide, has a hp composed above of small 

 cells of the anteroventral area, now the ectoderm or epiblast, 



^.n.p.- 



'Tich. 



,n.p. 



,nch. 



.mes.p. 



met. p. 



m.«.i, 



m.t.2 



^m-o. 3 



Fig. 481. — Transverse sections of embryos of Branchiostoma at stages 

 corresponding to Figs. 482 (i and 2). — After Hatschek. 



ach., Archenteron ; mes.p., pouch which will become first mesoblastic somite; n.p. neural plate- 



nch., rudiment of notochord. ' ' 



and at the sides and below of the cells of the crescent. The process 



continues by a rolling in over these lips, so that above ectoderm 



cells and at the sides the cells of 



the crescent become part of the 



lining of the cup. Meanwhile the 



embryo is lengthening, and the 



blastopore narrowing owing to 



the growth backwards of its h.c. 



anterior lip which, since it thus 



comes to cover the dorsal side, is 



known as the dorsal lip. The 



gastrula when it has thus been 



completed (Fig. 482) is elongate, 



with a small blastopore at the 



hinder end of its flat upper side. 



Its cavity is lined below and at the 



sides by endoderm and has for its 



ceiling in the middle a strip of 



cells which rolled in over the 



dorsal lip and will form the notO- Fig. 482.— Longitudinal sections of 

 , , J •,!_ • J r ii • embryos of Branchiostoma at 



chord, and on either side 01 this ^ 



a groove formed from the crescent 



cells. These cells are the mesoblast 



or the rudiment of the mesoderm. 



The cavity of the gastrula is 



the archenteron ; the blastocoele 



has been obhterated during the 



rcpre- 



stages later than that 

 sented by I'ig. 480, 7. 



I, Vertical section of the completed gastrula : 

 2a, vertical .ind 26, horizontal sections of 

 an early post-gastrula stajje in which only 

 the first mesoblastic somite has separatrtl. 



ach., arrhciitiTon ; blp., blastoiHirt- ; end., 

 endoderm ; h.c. region which will become 

 the anterior median pouch, or head 

 cavity; m.s., 1-3, mesoblastic somites; 

 n.p., neural plate ; nch., notochord ; ne., 

 neurenteric canal. 



