6 THE CELL 



side, they make a star, called the mother star or aster. While 

 the loose skeins are forming, delicate striae appear within the 

 achromatin, so disposed as to make their bases within the polar 

 field and directed toward one another, and their apices directed 

 toward the future new nuclei. These achromatin figures con- 

 stitute the nuclear spindle. They then arrange themselves into 

 two daughter wreaths, or asters, similar to the mother star. 

 At this juncture the cell protoplasm begins to divide by becom- 

 ing constricted in the center. The daughter stars are converted 

 into two new nuclei, in the inverse order to that by which the 

 original nucleus was broken up. Nuclear membranes and 

 nucleoli appear, the cell protoplasm divides into two new cells, 

 and the cycle is completed. 



Derivation of Tissues. The primary parent cell divides 

 into an innumerable mass of cells, which is called the blasto- 

 derm. The blastoderm soon divides into two more or less 

 distinct layers, an outer and an inner, named ectoderm and 

 entoderm, between which a middle layer develops, the meso- 

 derm. From these three primary layers all of the various tissues 

 of the body are later developed. (See Embryology.) 



