DEVELOPMENT 411 



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 

 rise to the epidermis, 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, bones, blood vessels, etc. The mesoblast 



FIG. 363. Transverse vertical sections of an egg, showing progressive stages of develop- 

 ment: a, notochord ; b, medullary furrow, becoming a closed canal in the last. 



thickens so as to form two parallel ridges running length- 

 wise of the germ, and leaving a groove between them 

 (medullary furrow and ridges) The ridges gradually 

 rise, carrying with them the epiblast, incline toward each 

 other, and at last unite along the back. So that we 

 have a tube of epiblast surrounded by mesoblast, which 

 is itself covered by epiblast. This tube becomes the 

 brain and spinal cord, whose central canal, enlarging 

 into the ventricles of the brain, tells the story of its 

 original formation. Beneath the furrow, a delicate 

 cartilaginous thread appears (called notochord} the 

 predecessor of the backbone. Meanwhile the mesoblast 

 has divided into two layers, except in the middle of the 

 animal, beneath the spinal cord, and in the head. One 

 of these layers remains attached to the epiblast, and 



