REPRODUCTION AND DEVELOPMENT 



565 



way that several distinct germ layers be- 

 come recognizable (Fig. 402); these are 

 named according to their positions relative 

 to one another. The outermost layer of cells 

 is then called the ectoderm; the innermost 

 layer which forms the wall of the archen- 

 teron is called the endoderm; the middle 

 layer located between ectoderm and endo- 

 derm is called the mesoderm. The stage of 

 development during which formation of 

 germ layers occurs is called the gastrula. The 



mass movements of groups of cells during 

 gastrulation are referred to as morphogenetic 

 movements, since they constitute the initial 

 step in transforming the simple blastula 

 into the rough outlines of the future body; 

 genesis of form is under way. Details of 

 gastrulation differ considerably in eggs of 

 different species, but the end result is es- 

 sentially the same in all. 



In most triploblastic animals, a cavity 

 soon forms within the mesoderm, and con- 



Germ layers 



Wall of 

 digestive tube " 



Body wall < 



Archenteron 



Neural fold 



Notochord 



Coelom 

 Endoderm 

 Splanchnic 

 mesoderm 



Somatic 



mesoderm 



Ectoderm 



Neural plate 



Ectoderm 



Endoderm 



Mesoderm 



Somites 



Heart 



Limb bud 



Coelom formation 



Neural tube 



Kidney tubules 

 Gonad 



OtII 



Neural tube stage 



Figure 402. Stereograms of parts of early stages in the development of a chordate embryo; 

 successively older stages are shown from germ layers to neural tube. Only the areas of mesoderm 

 in which the kidney tubules and gonads will develop are labeled, and not the actual developing 

 structures. 



