I.] SEGMENTATION. 25 



According to Oellacher, Studien aus dem Ins. f. Exper. Pathol. Vien. 1869, 

 p. I, sections taken through the centre of the germinal d ; sc at the beginning 

 of segmentation shew a somewhat uneven vertical furrow, ending below in a 

 small triangular space, where it joins a nearly horizontal furrow which meets 

 the surface of the egg at some little distance on either side of the vertical 

 furrow. It seems certain that these first-formed furrows do not include the 

 \vhole of the germinal disc, whose limits at this st:<ge are however uncertain. 

 In the later stages of segmentation not only do the first-formed segments 

 become further divided, but segmentation also extends into the remainder of 

 the germinal disc. Goette, Arckiv. Micr. Anat. x. 145, indeed maintains that 

 segmentation (at a later period) even involves material which is undoubtedly 

 white yolk. He describes nuclei as making their appearance in the upper 

 surface of the bed of white yolk, and the substance round them as rising up in 

 the form of papilhe, which are subsequently constricted off and set free as 

 supplementary segmentation masses. It is these, according to him, which give 

 rise to the formative cells spoken of in the next paragraph. He states that they 

 continue to be formed long after the commencement of incubation. We shall 

 return to this subject, when we come to discuss more fully the nature of the 

 process of segmentation, in describing the ova of other classes of vertebrates. 



Between the segmented germinal disc, which we may 

 now call the blastoderm, and the bed of white yolk on which 

 it rests, a space containing fluid makes its appearance. This, 

 gradually increasing in all dimensions, may be called the 

 segmentation-cavity. 



15. As development proceeds, segmentation reaches its 

 limits in the centre, but continues at the periphery, and 

 thus eventually the masses at the periphery become of the 

 same size as those in the centre. 



The distinction however between an upper and a lower 

 layer becomes more and more obvious. 



The masses of the upper layer arrange themselves, side 

 by side, with their long axes vertical ; their nuclei become 

 very distinct. In fact they form a membrane of columnar 

 nucleated cells. 



The masses of the lower layer, remaining larger than 

 those of the upper layer, continue markedly granular and 

 round, and form rather a close irregular network than a 

 distinct membrane. In them nuclei are either wholly absent 

 or at least not readily visible. 



It seems more probable that the nucleus is hidden than that it is really 

 absent. In the earliest stages of segmentation which we have examined when 

 the segments were still few in number, a very large proportion of both great 

 and small segments contained large well-formed nuclei. These nucleated 

 segments, which were found in both the superficial and deeper portions of the 

 disc, were invariably those in which the granules were for some reason or other 

 few and fine; in fact, wherever the granules were not sufficiently numerous to 

 render the body of the segment too opaque, there a nucleus could be detected. 



