DEVELOPMENT. 



199 



and vent (Figs. 165, 166). Some adult animals are little 

 more than such a sac. Hydra (Fig. 191), for instance, is 

 little different from a gastrula with tentacles, and one of 

 its relatives wants even these additions. 



Ordinarily, however, development goes much further. 

 From the two original layers arises, in various ways, a third 

 between them, making the three primitive germ-layers 

 epiblast) mesoblast, and hypoblast. This new layer is nec- 

 essarily in the primitive body -cavity, which it may fill 

 up ; or usually a new body-cavity is formed, in different 

 ways in different groups. In by far the great majority 

 of animals the digestive tract gets a new opening, which 

 usually becomes the mouth ; and the old mouth may 

 close, or serve only the functions of the vent. From this 

 point the development of each group must be traced in 

 detail. 



Development of a Hen's Egg. After the segmentation 

 the germinal disk divides into two layers, between which 

 a third is soon formed. The upper layer (epiblast) gives 



FIG. 1CT. Vertical Sections of an Egg, showing progressive stages of development: 

 a, uotochord ; 6, medullary furrow, becoming a closed canal in the last. 



rise to the cuticle, brain, spinal cord, retina, crystalline 

 lens, and internal ear. From the lower layer (hypoblast} 

 is formed the epithelium of the digestive canal. From 

 the middle layer (mesoblast) come all the other organs- 

 muscles, nerves, bones, etc. The mesoblast thickens so 

 as to form two parallel ridges running lengthwise of 

 the germ, and leaving a groove between them (medul- 

 lary furrow and ridges). 1 The ridges gradually rise, 

 carrying with them the epiblast, incline towards each oth- 

 er, and at last unite along the back. So that we have a 



