124: DEVELOPMENT OF THE IMPREGNATED EGG. 



in the egg ; and as soon as it is completed, the body of the foetus is 

 formed. The blastoderm is, in fact, the foetus in its earliest condition ; 

 for although its texture is at this time exceedingly simple, all the various 

 organs of the body will afterward be produced from it by the modifica- 

 tion of its different parts. The further process of formation is com- 

 paratively simple in some classes of animals, more complicated in 

 others ; and its general features are most easily understood by com- 

 mencing with the study of embryonic development as it takes place in 

 the frog. 



Formation of Organs in the Embryo. The egg of the frog, when dis- 

 charged and fecundated, is deposited in the water, enveloped in an elastic 

 cushion of albuminous matter. It is thus freely exposed to the light, 

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

 with an abundance of moisture and appropriate nutritious rftaterial. 

 Its development is distinguished accordingly by a character of great 

 simplicity ; since the whole of the vitellus is directly converted into the 

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

 sequently no complications of the formative process. 



The two blastodermic layers, above described, represent together the 

 commencement of the body of the embryo. They serve, however, for 

 the production of two different systems; and the entire process of their 

 development may be expressed as follows: The external blastodermic 

 layer produces the skin, the cerebro-spinal axis, and the organs of ani- 

 mal life ; while the internal layer produces the mucous membrane of 

 the alimentary canal, and the organs of nutrition. 



The first sign of advancing organization in the external blastodermic 

 layer shows itself in a thickening and condensation of its structure. 

 The thickened portion has the form of an elongated oval spot, termed 



the "embryonic spot" (Fig. 241), the wide 

 edges of which are somewhat more opaque 

 than the rest of the blastoderm. Inclosed 

 within these opaque edges is a narrower 

 colorless and transparent space, the " area 

 pellucida," and in its centre is a delicate 

 line, or furrow, running longitudinally from 

 front to rear, called the u primitive trace." 

 In the anterior portion of the area pel- 

 lucida, the substance of the blastoderm rises 

 up in such a manner as to form two nearly 



Diagrammatic view of the IM- parallel ridges or plates, which approach 

 PKEGNATED E o o , showing the each other, from side to side, over what 



SSSS^S-T 1 * e " ucl " a ' win be the dorsal '"i* " of the embr y' 



and are therefore called the " dorsal plates." 



Between them is included a groove, termed the "medullary groove." 

 The dorsal plates gradually meet each other and coalesce upon the 

 median line, thus converting the intervening groove into a canal. The 

 coalescence of the edges of the two dorsal plates takes place first in the 





