THE GERM-LAYERS. 



81 



of small cells over the outside will form the skin or outer layer 

 of the body- wall. 



The embryo very soon begins to swallow, through the blasto- 

 pore, the milklike lluid in which it floats, and to digest it with- 

 in the cavity of the archenteron. 



It is obvious that the embryo already shows a distinct differ- 



m 



C 



D 



FIG. 35. Diagrams of the early stages of development in the earthworm. A, accu- 

 rate drawing of the blastula, surrounded by the vitelline membrane (after Vej- 

 dovsky) ; B, blastula in optical section showing the large segmentation-cavity 

 (s.c.), and the parent-cell of the mesoblast (m.) ; C, later blastula, showing forma- 

 tion of mesoblast -cells ; D, flattening of the blastula preparatory to invagination ; 

 _E, the gastrula in side view; as the infolding takes place the two mesoblast- 

 bands are left at the sides of the body, in the position shown by the dotted lines ; 

 F, section of E along the line s-s, showing the mesoblast-bands and pole-cells. 



entiation of parts which perform unlike functions. In fact we 

 may regard the gastrula as composed of two tissues still nearly 

 similar in structure though unlike in function. One of these 

 consists of the layer of cells which forms the outer covering; 

 this tissue is known as the ectoblast (ec, Fig. 35). The second 

 tissue is the layer of cells forming the wall of the archenteron ; 

 it is called the entoblast (en). The ectoblast and entoblast to- 

 gether are known as the primary germ-layers. 



Meanwhile changes are taking place which result in the for- 

 mation of a third germ-layer lying in the segmentation-cavity 

 between the ectoblast and entoblast and therefore called the 

 mesoblast (w, Figs. 35, 36). In some animals the mesoblast 

 does not arise until after the completion of gastrulation. In 



