I.] SEGMENTATION. 23 



from the analogy of other forms, no doubt that in the 

 formation of the first two segments the original nucleus, 

 formed by the fusion of the male and female pronuclei, 

 becomes divided, and that a fresh division of the nucleus 

 takes place with the formation of each fresh segment. 

 Nuclei make their appearance moreover in the part of 

 the ovum immediately below that in which the segmen- 

 tation has already taken place ; these are in all proba- 

 bility also derived from the primitive nucleus. The 

 substance round some of these nuclei rises up in the 

 form of papillae, which are subsequently constricted off 

 and set free as supplementary segmentation masses ; 

 while some of the nuclei remain and form the nuclei 

 already spoken of as existing in the bed of white yolk 

 below the blastoderm in the unincubated egg. 



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. 



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 



