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 

 tip ; 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 (epiblasf) gives 



FIG. 167. Transverse Vertical Sections of an Egg, showing progressive stanes of de- 

 velopment : a, notochord ; b, medullary fnrrow, 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). 110 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 



