DEVELOPMENT OF THE IMPREGNATED EGG. 619 



larger and softer. The egg then has the form of a globular sac, the 

 walls of which consist of three concentric layers, in contact with each 

 other, namely : 1st, the structureless vitelline membrane inclosing the 

 whole ; 2d, the external blastodermic layer, composed of cells ; and 3d, 

 the internal blastodermic layer, also composed of cells. The cavity of 

 the egg is occupied by an albuminous fluid, absorbed from the exterior 

 and destined to serve as nutritious material. 



It is by this process that the simple globular mass of the vitellus is 

 converted into an organized structure. For the blastoderm, although 

 consisting of cells which are nearly uniform in size and shape, is never- 

 theless a distinct membrane, made up of anatomical elements ; and its 

 completion marks the first stage in the formation of the embryo. The 

 blastoderm is in fact the embryo in its primitive condition ; and although 

 its texture is at this time exceedingly simple, all the bodily organs are 

 afterward produced by the modification of its different parts. The 

 further process of formation is comparatively simple in some animals, 

 more complicated in others ; and its general features are most easily 

 understood by commencing with the study of development as it takes 

 place in the frog. 



Formation of Organs in the Frog. The egg of the frog, when 

 discharged and fecundated, is deposited in the water, enveloped in an 

 albuminous covering of gelatinous consistency. It is thus exposed to 

 the light, the air, and the moderate warmth of the sun's rays, and is 

 supplied with abundance of moisture and nutritious material. Its 

 development is distinguished by a character of great simplicity ; since 

 the whole, or nearly the whole, of the vitellus is directly converted into 

 the body of the embryo. There are no accessory organs, and conse- 

 quently no complications of the formative process. 



The two blastodermic layers above described represent the commence- 

 ment of the new organism. They serve, however, for the production 

 of different parts ; and the entire process of development may be con- 

 cisely expressed as follows : 



I. The external blastodermic layer produces the cerebro-spinal axis 

 and the epidermis of the general integument. 



II. The internal blastodermic layer produces the epithelium of the 

 alimentary canal and adjacent glandular organs. 



III. An intermediate layer, which subsequently appears between the 

 two, produces the vascular tissues, and thus completes the constitution 

 of the bodily frame. 



The first sign of advancing organization in the blastoderm shows 

 itself in a thickening and condensation of its structure. The thickened 

 portion has the form of an elongated spot, termed the "embryonic 

 spot " (Fig. Itt), the wide edges of which are more opaque than the 

 adjacent parts. Between these opaque edges is a narrower, colorless, 

 and transparent space the "area pellucida/' within which is a delicate 

 line, running longitudinally from front to rear, called the "primitive 

 trace." 



