DEVELOPMENT. 



199 



and vent (Figs. 166, 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, mesohlast, and hypoblmt. 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 diflEerent 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 {epihlast) gives 



FiQ. 1C7.— Transverse Vertical SeMions of an Egg, showing progressive stases of de- 

 velopment: a, notochord ; 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 {ky-poblast) 

 is formed the epitlieliuni of the digestive canal. From 

 the middle layer (mesohlast) come all the other organs — 

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

 as to form two parallel ridges running lengtiiwise of 

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

 lary furrow and ridgei).^^" 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 



