770 HANDBOOK OF PHYSIOLOGY. 



ning across the disc dividing it into two; it does not extend across 

 the whole breadth. A second furrow, at right angles, cutting the first 

 a little eccentrically, next appears, and the disc is thus cut into four 

 quadrants. The furrows do not extend through the whole thickness of 

 the disc, and the segments are not separated out on the lower aspect. 

 The quadrants are next bisected by radiating furrows, and the disc is 

 thus divided into eight parts. The central portion of each segment is 

 now cut off from the peripheral furrow, so that a number of smaller 

 central and larger peripheral portions result. As the primary division 

 was eccentric and the succeeding followed the same plan, there results 

 a bilateral symmetry ; but the relation of the axis of symmetry and the 

 long axis of the embryo is not known. Eapid division of the segments 

 by furrows in various directions now ensues, and the small central por- 

 tions are more rapidly broken up than the larger, and therefore become 

 more numerous. During this superficial segmentation a similar process 

 goes on throughout the whole mass, and division goes on not only by 

 vertical but also by horizontal furrows. The result of this process of 

 segmentation is that the original germinal disc is cut into a large num- 

 ber of small rounded protoplasmic cells, small in the centre, larger to 

 the periphery, and that the superficial cells are smaller than those be- 

 low : the two original layers of the blastoderm are thus early represented. 

 The process of segmentation proceeds at the periphery of the ger- 

 minal disc, and at the same time further division of the cells at the 



Fig. 461. Vertical section of area pellucida and area opaca (left extremity of figure) of 

 blastoderm of a fresh-laid egg (unincubated). S, superficial layer corresponding to epiblast; 

 D, deeper layer, corresponding to hypoblast, and probably in part to mesoblast; M, large 

 "formative cells," filled with yolk granules, and lying on the floor of the segmentation cavity; 

 A, the white yolk immediately underlying the segmentation cavity. (Strieker.) 



centre proceeds. The nucleus of the original cell divides coincidently 

 with the protoplasm, and so it comes that the protoplasmic masses are 

 nucleated; and besides this, nuclei derived from the original nucleus 

 are found in the ovum below the area of segmentation, and from these 

 by the protoplasm which surrounds them being constricted off with 

 them, supplementary segmentation masses come to be formed. The 

 blastoderm is thus formed as the result of segmentation, and between it 

 and the subjacent white yolk is a cavity containing fluid. The segmen- 

 tation having been completed toward the centre, although it still pro- 

 ceeds at the periphery, the superficial layer of the blastoderm becomes 



