56 



AMPHIOXUS. 



blastopore is effected entirely by growth backwards of Lte 

 anterior lip, then it is evident from a comparison of Figs. 17, 18, 

 and 19 that the blastopore originally occupies almost the whole 

 of what will afterwards be the dorsal surface of the larva; 

 while the outer or convex surface of the young gastrin 1 a (Fig. 17) 

 corresponds to the ventral surface, and perhaps also to the 

 anterior end of the larva. If these determinations are correct, 

 Figs. 15, 16, and 17 show that the lower pole of the blastula 

 corresponds to the dorsal surface of the larva, and the upper 

 pole to its ventral surface. 



Before leaving the gastrula the cells of the two layers, epi- 

 blast and hypoblast, should be noticed more fully. The epiblast 



Fig. 19. 



Fig. 20. 



Figs. 19 and 20. — The fully formed gastrula of Amphioxus. 

 x 220. (After Hatschek.) 



Fig. 19.— The gastrula bisected vertically : the left half is represented, as seen from 

 the inner surface. 



Fig. 20.-The gastrula bisected horizontally : the ventral half is represented, as seen 

 from above. 



BP, blastopore. Gr, archenteron. PC, polar mesoblast cell. 



(Figs. 19 and 20) is a single layer of very short columnar or 

 almost cubical cells ; at about the stage represented by Fig. 18 

 these cells develop on their outer surfaces rlagella or lash- 

 like processes, one from each cell, by which the embryo is caused 

 to rotate within the vitelline membrane. These flagella persist 

 during the greater part of the pelagic existence of the embryo, 

 but are not represented in the figures given here. 



The hypoblast is a single layer of elongated columnar cells, 

 with nuclei near their inner ends. At the lip of the blasto- 

 pore the epiblast and hypoblast cells are necessarily continuous 

 with one another ; in the mid-ventral line the two polar meso- 

 blast cells render the transition an abrupt one ; but all round 



