Segmentation of Egg. 



129 



of cells, two or three deep, is formed. Of these layers the 

 outermost is soon separable for the deeper layers, and between 

 it and them a cavity — the blastocOBl — appears. The outer 



Fig. 65. — Karyokinesis of a typical tissue-cell (epithelium of Salamander). (After 

 ' Flemming and Klein.) 



The series from a to i represents the successive stages in the movement of the chromatin 

 fibres during division, excepting g, which represents the *' nucleus-spindle" of an 

 egg-cell, hy resting nucleus ; d, wreath-form ; E, single star, the loops of the wreath 

 being broken; f, separation of the star into two groups of U-shaped fibres; h, diaster 

 or double star ; i, completion of the cell-division and formation of two resting nuclei. 

 In G the chromatin fibres are marked a, and correspond to the "equatorial plate ; " 

 ^, achromatin fibres forming the nucleus-spindle ; c, granules of the cell-protoplasm 

 forming a "polar star." Such a polar star is seen at each end of the nucleus-spmdle, 

 and is not to be confused with the diaster h, the two ends of which are composed 

 of chromatin, 



layer thus definitely established is known as the epiblast (Fig. 

 ^^) ; the remaining cells may be termed, for the present and 



Fig. 66. — Blastoderm of Hen's egg at time of laying. (From Marshall.) 



E, epiblast ; h, hypoblast ; n', nucleus in yolk, round which a cell will be formed later ; 



ZL, lower layer cells ; bv, subgerminal space ; y, yolk. 



collectively, the lower layer cells. These are increased by 

 new cells out of the yolk (n. Fig. 66). 



K 



