FIG. 13. Mesoderm formed by pouches from entoderm after, gastrulation. a, primi- 

 tive gut; bp., blastopore; cor, body cavity, formed from pockets of the archenteron; ec., 

 ectoderm; en., entoderm; m., mesoderm; m.so, body-wall mesoderm; m.sp., visceral 

 mesoderm; s.c., segmentation cavity. 



Questions on figures 12 and 13. Enumerate the three modes of meso- 

 derm formation figured here. In which type may the mesoderm be identi- 

 fied most early in the embryonic development? By comparing with other 

 texts determine in what groups of animals the mesoderm is formed as in 

 Fig. 13. What are the differences between A and B of Fig. 13? 



groups of animals. It may originate (i) from the multipli- 

 cation of a few special cells which, before invagination, be- 

 come distinct from those that are to form ectoderm and ento- 

 derm (Fig. 12, A and B, m} ; (2) by means of isolated, wan- 

 dering cells budded from the other two layers, particularly the 

 entoderm (Fig. 12, C ' , c) ; or (3) from entoderm, in the form 

 of pouches or of solid buds of cells which arise from the walls 

 of the archenteron and extend into the segmentation cavity 

 (Fig. 13, m). In some instances there may occur a combina- 

 tion of these methods. 



56. Coelom. When the mesoderm develops by the last 

 mentioned method, i. e. by the evagination of the wall of the 

 primitive gut (Fig. 13, m), we see a p%ir of 'folds, or a series of 

 pockets, the cavities of which are at first continuous with the 

 archenteron, but later become separate from it and entirely 

 surrounded by the mesodermic layers. The outer wall of the 

 mesodermic pouches joins the ectoderm and forms a body 

 wall, and the inner applies itself to the entodermal wall of the 

 gut. The space between is the ccelom or body cavity. When 



