XVII 



PEOTOCHOEDATA 



617 



face, appears semicircular, the blastopore is really wide in front and 

 narrow behind ; for behind it is a groove between the two columns 

 of muscle and mesenchyme cells on each side, the mesenchyme cells 

 being above and external as regards the muscle cells. 



The overarching rim of cells, which is the anterior lip of the 

 blastopore and corresponds to the region marked x in the gastrula 



tnch 



sup 



FIG. 446. Illustrating the closure of the blastopore in the gastrula of t'l/ntt/in /m,-titn. 



(After Conkliu.) 



A, dorsal view of advanced gastrula, showing the blast opore as a minute pore at- (In 1 hinder end of 

 tin- neural plate. 13, posterior view of advanced gastrnla, showing the neural folds. C, lateral view of 

 advanced gastrnla, showing the rotation of the mesodennal lands into a vertical posit ion liy the up- 

 growth of the ventral lip of the blastopore and the division of the inesenchynie into a superior and an 

 inferior mass, blji, disappearing blastopore ; iiicli.i/if, interim- mass of inesenchynie ; HH//.M//S superior 

 mass of mesenchyme ; ma, mesodennal band; ii.f, neural fold ; n.ji, neural plate. 



of Amphioxus, continues its growth backward till it covers three- 

 quarters of the dorsal face of the embryo. Then the groove between 

 the lateral walls of muscle and mesenchyme becomes tilted upwards 

 till it becomes vertical, and the mesenchyme cells are placed in 

 front of the muscle cells. This change is due to the sudden up- 

 growth of the posterior and ventral lip of the blastopore, the region 

 corresponding to the region // in the gastrula of Amphioxus. The 



