12!: 



Elementary Zoology. 



passing over of half of the chromosomes to the daughter 

 nucleus; but each chromosome splits longitudinally into two 

 parts, and the result is that each of the two nuclei formed out 

 of the original single nucleus contains exactly the same number 

 of half chromosomes. The remaining divisions of the cell 

 go on precisely the same way. 



In the fowl's egg the division is limited to the germinal 

 disc. The first furrow formed runs across the disc, but does 



Fig. 64. — Successive stages in the division of tlie ovum, or egg-cell, of a worm. 

 , (After Strasburger.) 



a to ^ show the changes taking place in the nucleus and surrounding cell-contents, which 

 result in the first segmentation of the ovum at e : _/" and f; show a repetition of these 

 changes in each of the two resulting cellsj leading to tiie second segmentation 

 stage at h. 



not quite reach its margin on either side ; the second furrow 

 is at right angles to this, and in the same way does not reach 

 the edge. Finally, the disc is broken up by a series of sub- 

 sequent furrows into a mosaic, which is limited to the central 

 region of the germinal disc, and is slightly excentric. At first 

 this cap of cells, thus formed, is a single layer in thickness, the 

 segmentation being confined to the superficial layer of the 

 germinal disc. Later on the segmentation takes place also in 

 the deeper layers of the germinal disc, so that ultimately a cap 



