176 Harry Lewis Wieman. 



have described a mechanism which may have the same significance. 

 Here the direct division is said to be brought about by the con- 

 stricting power of a ring-shaped centrosome. 



The fact that in both the germ and nurse cells, the daughter 

 nuclei are of approximately equal size, indicates the presence of 

 a division mechanism of great precision and accuracy. The power 

 or force involved in karyokinesis therefore, may not lie in the 

 visible structures — centrosome, spindle, chromosomes, etc. — but 

 in some invisible factor that is a property of the living protoplasm. 



Mitosis and amitosis are often regarded as representing anti- 

 thetical conditions, but, as a matter of fact, these two methods of 

 cell division really stand for the extremes of a graded series. A 

 simple type of amitosis is that shown in the nurse cells, where there 

 is no evidence of any division apparatus. The nucleus undergoes 

 a simple constriction and divides. In the case of the germ cells, 

 the presence of the nucleolus, its exact division and the occurrence 

 of the surrounding pale area, point to what might be called a 

 higher type of amitosis. Another advance is seen in the germ 

 cells of amphibia, where the constriction centrosome divides the 

 nucleus (Meves and Benda). In many of the so-called mitotic 

 division figures of the lower forms, the spindle is far from conspic- 

 uous and in many cases is represented by only a few strands of 

 achromatic substance. In the division figure of the macronucleus 

 of Spirochona and Actinosphaerium as figured by R. Hertwig 

 ('79, '84), the spindle and equatorial plate are formed inside of 

 the nuclear membrane. In Spirochona a hemispherical ' ' end plate" 

 or ''pole plate" is situated at either pole of the spindle. Hertwig 

 claims that these arise by a division of a large nucleolus whose 

 behavior reminds one of the large chromatin nucleoli of the germ 

 cells of Leptinotarsa. Keuten ('95) has demonstrated the origin 

 of similar pole plates from nuceoli in Euglena and Schaudinn 

 ('95) in Amoeba. Euglena presents a very primitive type of 

 mitosis, spindle fibers being scarcely recognizable. 



Among the metazoa, examples are to be found in which the 

 mitotic figure is equally primitive, as in certain aphids, where 

 according to Tannreuther ('07) no chromatic spindle occurrs in 

 the maturation division of the egg. Definite chromosomes of 



