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. 167.—Vertical Sections of an Egg, showing progressive stages of-development: 
a, notochord; b, 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). 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 
