DEVELOPMENT OF THE IMPREGNATED EGG. 723 



tion of an embryo has commenced. It takes place in all species of 

 animals, although it varies in detail according to the special constitu- 

 tion of the egg, and the presence or absence of accessory parts. In all 

 the mammalia, as well as in many of the invertebrates, where the vitellus 

 is very small, and where the body of the embryo immediately after its 

 formation is to be supplied with nourishment from without, the process 

 is that described above. In the birds, in scaly reptiles, and in many 

 fish, where the vitellus or yolk is of large size, and contains additional 

 nutritive matter, segmentation takes place only in a thin layer which 

 occupies the surface of the great mass of the yolk ; and, beginning at 

 one spot, extends thence from within outward, so that it advances more 

 rapidly at the centre of the segmenting region than at its periphery. 

 But in all cases segmentation of the vitellus is the first change to occur 

 in the process of development, and its result is alwaj^s the same, namely, 

 to divide the vitellus, which was at first of uniform texture throughout, 

 into a great number of minute bodies, which soon present the character 

 of animal cells. 



Blastoderm, or Germinal Membrane. The cells which are formed, in 

 the manner above described, by the segmentation of the vitellus, become 

 more closely packed as they increase in number ; and finally, by their 

 mutual contact, and adhesion at their adjacent edges, they serve to form 

 a continuous organized membrane, known as the germinal membrane 

 or blastoderm. 



During the formation of this membrane, moreover, the egg, while 

 passing through the Fallopian tube, increases in size. The albuminous 

 matter with which it is enveloped becomes liquefied ; and, being absorbed 

 by endosmosis through the vitelline membrane, furnishes the material 

 for the more solid and extensive growth of the newly-formed structures. 

 A certain quantity of fluid also accumulates in the central cavity of the 



egg- 



The next change which takes place consists in the division or splitting 

 of the blastoderm into two layers, which are known as the external and 

 internal blastodermic layers. They are both still composed exclusively 

 of cells ; but those of the external layer are smaller and more compact, 

 while those of the internal are larger and less consistent. The egg then 

 has the form of a globular sac, the walls of which consist of three con- 

 centric layers, tying in contact with and inclosing each other, namely : 

 1st, the structureless vitelline membrane on the outside; 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 only of cells which are nearly uniform in size and shape, is 

 nevertheless an organized membrane, made up of anatomical elements. 

 It is the first sign of distinct organization which makes its appearance 



