FROM SIMPLE CELL TO COMPLEX ANIMAL 4I 



(i) from the multiplication of a few special cells which, before 

 invagination, in early cleavage stages, become distinct from 

 those that are to form ectoderm and entoderm (Fig. 14, A and 

 B, m)\ (2) by means of isolated, wandering cells budded from 

 the other two layers, particularly the entoderm (Fig. 14, 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. it,, m). In some instances 

 there may occur a combination of these methods. 



Fig. 15. 



Pig, 15. Mesoderm formed by pouches from entoderm after gastrulation. A and £, early 

 and later stages m formation of mesoderm and coelom. a, primitive gut; bp., blastopore; ca, body 

 cavity, formed from pockets of the archenteron; «c., ectoderm; en., entoderm; m., mesoderm; m.so. 

 body-wall mesoderm; m.sp., visceral mesoderm; 5.C.. segmentation cavity. 



Questions on figures 14 and 15. — Enumerate the three modes of mesoderm 

 formation figured here. In which type may the mesoderm be identified most early 

 in the embryonic development? By comparing with other texts determine in 

 what groups of animals the mesoderm is formed as in Pig. 15. 



58. Coelom, or Body Cavity. — When the mesoderm develops 

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

 wall of the primitive gut (Fig. 15, m), we see a pair 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 coelom or body cavity. 

 When the mesoderm arises as a solid mass, instead of a pocket, 

 the body cavity is formed by the splitting of the mass into an 

 inner and an outer portion. When the coelom is formed by 

 several pockets the cavities of these may ultimately coalesce, 



