ORIGIN OF THE GERMINAL LAYERS. 



353 



Brachiopoda. The walls of a pair of outgrowths form the whole of the 

 mesoblast. 



Chaetognatha. The mesoblast arises in the same manner as in the 

 Brachiopoda (fig. 209). 



Echinodermata. The lining of the peritoneal cavity is developed from 

 the walls of outgrowths of the archenteron, but the greater part of the meso- 

 blast is derived from the amoeboid cells budded off from the walls of the 

 archenteron (fig. 210). 



ME 



Mp. Pld. 



FIG. 210. LONGITUDINAL SECTION THROUGH AN EMBRYO OF CUCUMARIA 

 DOL1OLUM AT THE -END OF THE FOURTH DAY. 



Vpv. vaso-peritoneal vesicle; ME. mesenteron; Sip., Ptd. blastopore, proc- 

 todjeum. 



Enteropneusta (Balanoglossus). The body cavity is derived from two 

 pairs of alimentary diverticula, the walls of which give rise to the greater 

 part of the mesoblast. 



Chordata. Paired archenteric outgrowths give rise to the whole meso- 

 blast in Amphioxus (fig. 211), and the mode of formation of the mesoblast in 

 other Chordata is probably secondarily derived from this. 



3. The cells which will form the mesoblast become marked 

 out very early, and cannot be regarded as definitely springing 

 from either of the primary layers. 



Turbellaria. Leptoplana (fig. 212), Planaria polychroa (?). 



Chsetopoda. Lumbricus, &c. 



Discophora. 



It is very possible that the cases quoted under this head ought more 

 properly to belong to group i. 



4. The mesoblast cells are split off from the epiblast. 



Nemertea. Larva of Desor. The mesoblast is stated to be split off 

 from the four invaginated discs. 



B. III. 23 



