42 



GENERAL CONCEPTS 



Figure 3.6. Photomicrograph of the mitotic apparatus isolated from dividing cells of 

 a sea urchin embryo. Each mitotic apparatus includes spindle fibers, asters and chromo- 

 somes. A metaphase figure appears in the upper right and two anaphase figures below. 

 (Courtesy of Daniel Mazia.) (Villee: Biology.) 



together that individual ones cannot be distinguished. The reduplication 

 of the chromosomes has occurred previously and in many species ot 

 animals the double nature of each chromosome is apparent. 



Early in prophase the centriole, a small granular structure in the 

 cytoplasm, divides and the daughter centrioles migrate to opposite sides 

 of the cell. Between the separating centrioles a spindle forms. The 

 spindle is composed of spindle fibers, protoplasmic threads arranged like 

 two cones base to base (Fig. 3.6). The spindle is broad at the center or 

 equator of the cell and narrows to a point at either end or pole. The 

 spindle is not some optical artifact but a definite structure composed of 

 protoplasm that is denser than the surrounding protoplasm. With a 

 microneedle attached to a micromanipulator the spindle can be moved 

 as a unit from one part of the cell to another. At the end of prophase, 

 the centrioles have divided and gone to the opposite poles of the cell, the 

 spindle has formed between them and the chromosomes have become 

 short and thick. 



Metaphase. When the chromosomes are fully contracted and ap- 

 pear as short, dark-staining rods, the nuclear membrane disappears and 

 the chromosomes line up in the equatorial plane of the spindle. The 

 short period during which the chromosomes are in this equatorial plane 

 is known as the metaphase. This is much shorter than the prophase; 

 although times for different cells vary considerably, the prophase lasts 



