Origin of tailed Chordate Stage 



55 



the exterior by means of the deuterogenetic channels of the 

 coelom or renal apparatus. 



The same relations between the two regions probably hold 

 good for the amniotes, though in them experimental evidence is 

 obtained less easily. 



Now it is conceivable that having attained the stage of a 

 coelenterate the gastrula may take very different courses of 

 development. For instance, the whole deuterogenetic centre 

 might continue active, and thus produce a long cylindrical addition 

 to the spherical gastrula, Fig. 31 A. Or it might cease almost 



Fig. 31. Diagrams of sagittal sections of stages in the development of a craniate 



chordate, showing the growth centres. 



A, Gastraea stage; B, Balanoglossus stage; G, tailed chordate stage. 



as soon as formed, in which case there would be no great alteration 

 from the radially symmetrical form, Fig. 31 B. The organism 

 might retain more or less perfectly its radially symmetrical form. 

 This would be due to the dying out altogether and at once of the 

 deuterogenetic centre of activity. Or a portion of the deutero- 

 genetic region might remain for a longer time active while the 

 rest dies out earlier, Fig. 31 C. The portion that remains active 

 might be that forming one border of the blastopore, while all 

 round the rest of the blastopore the activity dies out. The 

 latter part therefore would be left stationary while the active 

 part grew back. 



It may be suggested that deuterogenesis in the embryo 

 is a geometrical consequence following gastrulation (which may 



