



58 PRINCIPLES OF ANIMAL BIOLOGY 



divides all the different components, so that the resulting two chromo- 

 somes have the same pattern as the original one. 



Anaphase. — From their position in the equatorial plate the two 

 chromosomes, formed from one, move or are drawn toward opposite 

 ends of the spindle. This stage is known as the ana-phase. The shapes 

 of the chromosomes often indicate that they are being pulled. Thus, in 

 Fig. ?>bG, the long chromosomes could be given their V shape by being 

 pulled from their middle points toward the centrioles. Moreover, some 

 of the so-called fibers extending out from the centrioles may often be 

 seen to attach to the chromosomes at these points. Consequently, the 

 fibers are often thought of as pulling the duplicated chromosomes apart. 

 Whether they actually pull or not is uncertain. The fibers may be only 

 lines of flow in the protoplasm, that is, courses along which the fluid 

 protoplasm is moving. Whatever causes this flow could drag the chromo- 

 somes along. If the middle parts of the long chromosomes were caught 

 in this current, the characteristic V form of such chromosomes would 

 still result. 



Whatever the cause of their movement, one chromosome of each 

 pair of duplicates goes to each end of the spindle {H). Here they collect 

 in two close groups (/), ready to form two new nuclei. In the meantime 

 the cytosome narrows between the retreating groups of chromosomes 

 (//, 7) and finally constricts in a sharp furrow (/) which eventually cuts 

 the cell completely in two (J). In many cells, about this time, the 

 centriole divides in two, as if in preparation for the next division (7), so 

 that during the whole ensuing interphase the centriole is double. 



This separation of the daughter chromosomes has as important a 

 consequence as does the longitudinal duplication of each one. ' The 

 chromosomes are of different kinds; they contain different things. Each 

 cell possesses a complete set of the different kinds of chromosomes. The 

 accurate separation of the sister chromosomes, one going to each pole 

 of the spindle in the anaphase, insures that the two daughter cells will 

 likewise have a complete set of chromosomes. All the cells of a multi- 

 cellular animal thus have identical chromosomes in them. 



Telophase. — The remainder of the process of cell division consists of 

 th(^, restoration of the chromosomes to the diffuse state in whicli they 

 existed before division began, and the disappearance of all remnants of 

 the divisi(m apparatus from the cytosome. The chromosomes become 

 diffuse either by becoming filled and distended with a fluid or by spinning 

 out their chromatin into fine, perhaps branching, threads, as explained 

 on page 26. Some particulars of this process are given later. By either 

 method the chromatin comes to be scattered in irregular knots or strands, 

 giving the appearance of a network. A membrane is formed about the 

 whole grou]) of chromosomes (./) and the reconstruction of the nuclei 



