48 EISEN. [Vol. XVII. 



Contractile Fibers. 



This class of fibers is of sufficient importance and interest 

 to be treated of separately. Their origin is different from that 

 of other fibers, and they also appear, partially at least, to con- 

 sist of a different kind of protoplasm. They are the only fibers 

 which connect directly with the somosphere and which thus 

 penetrate the centrosphere. As to the actual beginning of 

 the contractile fibers, there are no satisfactory observations, 

 and we do not know if the narrow thread in the centrosphere 

 orginates previous to the part outside of the centrosphere, but 

 I am inclined to think that this thin thread is formed after the 

 balance of the fiber. The number of contractile fibers is the 

 same as the number of the chromosomes, as there is a special 

 fiber for each chromosome. These fibers show from the begin- 

 ning a different structure from any of the other classes of fibers, 

 being from their first appearance beaded {Figs. 41-54). Each 

 fiber is composed of a thin outer sheath which is too small 

 to allow of its structure being perceived. Inside of this sheath 

 the protoplasm of the fiber is distinctly beaded in a manner 

 that greatly reminds us of the cytoplasmic arrangement of the 

 muscle fiber. These beads are not always of the same size, 

 those in the middle of the fiber often being the largest. There 

 is never more than one row of beads, which begins on the outer 

 side of the centrosphere and extends to the immediate vicinity 

 of the chromosome. Just before the fiber reaches the chromo- 

 some it divides into two tiny branches or arms, each arm con- 

 necting with different points of the chromosome. As soon as 

 the chromosome has reached the equator the contractile fiber 

 begins to contract, becoming thicker and shorter as well as 

 darker staining. When the confluent umbrella stage is reached 

 the fibers lose their intense staining capacity and finally disap- 

 pear. As regards this disappearance there are several conjec- 

 tures possible. Either the fibers are entirely absorbed and 

 changed into cytoplasm, or they are condensed into accessory 

 archosomes which reappear on the cytoplasmic membrane, at 

 this time re-forming at a short distance from and around the 

 nucleus. Or we may even suppose that they follow the central 



