THE EARTHWORM 



215 



is to decrease the blastula cavity and to make another cavity 

 with a wide opening to the outside (Fig. 105, _#, F). This is 

 the gastnda (little stomach) stage. It is appropriately called 

 so, because at this time the embryo first begins to take food 

 in the egg-capsule ; the food is digested in the cavity formed 

 from the outside. In the gastrula it is possible to distinguish 

 two layers of cells, both of which we know came originally 



E 



FIG. 105. Early Stages of Development of Earthworm. Much enlarged 



A, blastula, surface view. B, blastula, section: s.c., blastula cavity ; m, first cell 

 of mesoderm. C, blastula, later stage. D, blastula, flattening of future 

 endodermal cells. E, gastrula, in side view : ec, ectoderm ; en, endoderm ; 

 s-s, line of section for F. F, gastrula, cross section : ar, archenteron 



(From Sedgwick and Wilson's General Biology) 



from a single layer; -these two layers are called ectoderm 

 (Fig. 105, E, ec) and endoderm (Fig. .105, E, en). 



While the embryo is still in the blastula stage a single cell 

 (Fig. 105, B, m) can be identified as one which later develops 

 into a row of cells on either side, and finally into a mass of 

 tissue on either side, to which we give the name mesoderm 

 (Fig. 105, F). Many animals pass through these stages ; hence 

 the use of the terms ectoderm, endoderm, and mesoderm, as 



