618 REPRODUCTION. 



space existing at certain points between them and the vitelline mem- 

 brane is occupied by a transparent fluid. 



The process thus commenced goes on by the repeated formation of 

 furrows in various directions, dividing the four separated portions suc- 

 cessively into eight, sixteen, thirty-two, sixty-four, and so on ; until 

 the vitellus is converted into a mulberry-shaped collection of nearly 

 spherical nucleated bodies, resulting from its continued subdivision 

 (Fig. 116, C, D). These bodies are termed the " vitelline spheres." 

 They are of firmer texture than the original vitellus ; appearing to 

 increase in consistency as they multiply in numbers and diminish in size. 

 They become at last so abundant as to assume by mutual compression 

 the polygonal form (Fig. IT 6, E), lying in close contact with each other 

 immediately beneath the vitelline membrane, and surrounding a central 

 space filled with transparent fluid. They are thus converted into a 

 layer of cells, enclosing the original cavity of the egg, and enveloped 

 by the vitelline membrane (Fig. It 6, E). 



The segmentation of the vitellus is the primary act in the develop- 

 ment of the impregnated egg, and the sign that the formation of an 

 embryo has commenced. It takes place in all species of animals, 

 although varying in detail according to the special constitution of the 

 egg and its accessory parts. In all mammalia, as well as in many 

 invertebrates, where the vitellus is very small, and where the body of 

 the embryo immediately after its formation is supplied with nourish- 

 ment from without, the process is that described above. In birds, in 

 scaly reptiles, and in many fish, where the vitellus is large and contains 

 additional nutritive matter, segmentation takes place only in a thin 

 layer on the surface ; and, beginning at one spot, extends outward, 

 advancing 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 in the process of development, and has always the same result, 

 namely, to divide the vitellus into a great number of minute bodies, 

 which present the character of cells. 



Blastoderm, or Germinal Membrane. The cells formed, in the man- 

 ner above described, by the segmentation of the vitellus, become more 

 closely packed as they increase in number ; and finally, by mutual con- 

 tact arid adhesion at their edges, they form a continuous organized 

 membrane, known as the germinal membrane or blastoderm. 



During the formation of this membrane, the egg, while passing- 

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

 with which it is enveloped is liquefied and absorbed by the vitelline 

 membrane, furnishing material for the growth of the newly-formed 

 structures. A certain quantity of albuminous fluid also accumulates 

 in the central cavity of the egg. 



The next change consists in the appearance in the blastoderm of two 

 separate layers, 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, those of the internal 



