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HANDBOOK OF PHYSIOLOGY. 



sibly into it. The first change consists in the appearance of a furrow 

 running 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 

 portions are more rapidly broken up than the larger, and therefore be- 

 come more numerous. During this superficial segmentation a similar 

 process goes on throughout the whole mass, and division goes on not 



FIG. 443. Vertical section of area pellucida and area^ opaca (left extremity of ^figure) of 

 blastoderm of a fresh laid egg (unincubated). 

 deeper layer, corresponding to hypoblast, and 

 cells, "filled with yelk granules, and lying on ux^^v^,* ^ . ~~ 

 yelk immediately underlying the segmentation cavity (Strieker), 



only by vertical but also by horizontal furrows. The result of this pro- 

 cess of segmentation is that the original germinal disc is cut up into 

 a large number of small rounded protoplasmic cells, small in the centre, 

 larger to the periphery, and that the superficial cells are smaller than 

 those below: the two original layers of the blastoderm are thus early re- 

 presented. 



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

 nal disc, and at the same time further division of the cells at the cen- 

 tre proceeds. The nucleus of the original cells divides coincidently with 

 the protoplasm, and so it comes that the protoplasmic masses are nucle- 

 ated; 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 blasto- 

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

 the subjacent white yelk is a cavity containing fluid. The segmentation. 



