52 GENERAL EMBRYOLOGY 



the centrosomes, forming in this stage a superficial sheath 

 around a central portion of the spindle, and are hence termed 

 the mantle fibers. The central core of the spindle seems in 

 many cases to be formed largely from the remainder of the 

 nuclear linin, though in other cases this is formed in the same 

 way that the asters are, and from cytoplasmic materials. Thus 

 the amphiaster is usually of mixed origin, nuclear and cyto- 

 plasmic, though in some cases the spindle at least seems to be 

 wholly nuclear (linin); the asters are always cytoplasmic in 

 origin. All these fibers form definite threads, enlarged as 

 compared with the original linin reticulum. 



The definite formation of the chromatic portion of the 

 nucleus into chromosomes and the achromatic substance into 

 the amphiaster marks the termination of the first phase of 

 mitosis which is known as the prophase. During the prophase 

 there has occurred the actual division of only the centrosome 

 and centrosphere; the other important changes have been 

 preparatory to further divisions the dissolution of the nuclear 

 membrane, the enlargement and rearrangement of the chro- 

 matin granules, the formation of definite chromosomes, and the 

 establishment of the achromatic figure. We should remember 

 that the nucleolus meanwhile has fragmented and, together 

 with a large or small amount of chromatin which is not formed 

 into chromosomes, has passed out into the cytoplasm and 

 disappeared. 



The arrangement of the materials forming the achromatic 

 figure is evidently the result of certain tensions within the cell, 

 the effect of which is first to draw the chromosomes, until now 

 distributed irregularly, into a circle about the equator of the 

 spindle. When in this position the chromosomes are said to 

 form the equatorial plate (Figs. 20, F, G;21,F). This phase of 

 mitosis is also in general preparatory to actual division but it 

 is carried on after the division mechanism is completely 

 established. This period of division is known as the mesophase 

 (Lillie). 



Following the mesophase is the metaphase. The chief event 

 of this phase is the longitudinal splitting or division of each 



