206 



PRINCIPLES OF ANIMAL BIOLOGY 



the groove now in the form of a tube is completely separated from the 

 endoderm {B). The two tubes thus formed are the mesoderm, and the 

 slender openings in them constitute the body cavity, or coelom. In later 

 stages of development the tubes expand, as in C, shown black. One 

 side becomes a thin layer of cells applied to the digestive tract, while 

 the other side lines the inside of the ectoderm. 



ECTODERM- 



ALECITHAL 



BLASTOCOELE 



ENDODERM 

 APCHENTERON 



MILDLY 

 TELOLECITHAL 



■BLASTULA GASTRULA' 



Fig. 177. — Gastrulation of embryos, in relation to the quantity and distribution of yolk in 



them. 



In the frog the mesoderm is formed simultaneously with the endo- 

 derm — indeed, almost before endoderm — <iuring gastrulation. The cells 

 which turn in over the dorsal rim of the curved blastopore, in its middle 

 portion, form the mesoderm directly (Fig. 180). A band of these cells 

 migrates forward from the blastopore, above the archenteron, to the 

 front end of the future embryo. In late stages of gastrulation, cells 



^.«s 



'':<^. 





Fig. 178. — Gastrulation in, frog, external view. 



invaginated at the lower margin of the (now circular) blastopore also 

 contribute to the mesoderm. The endoderm below the main sheet of 

 mesoderm is in the form of an open trough. The upper rims of this 

 trough (a) were originally continuous with the edges of the mesoderm, 

 but they break loose and curve up under the mesoderm. They meet 

 at the top, to enclose a tube which becomes the intestinal tract. 



Subsequent Development of the Vertebrates. — The three layers of 

 cells, ectoderm, endoderm, and mesoderm, are often called gerin layers. 

 They are so designated because certain organs are normally derived 



