KARYOKINESIS. 43 



The spindle greatly increases in length from the prophase to the telophase. 

 R. Hertwig ('99) has observed that in Actinosphcerium the spindle more than 

 doubles in length during this period, and m Crepidula the lengthening is nearly as 

 great. The shape of the spindle varies greatly from prophase to telophase, being 

 largest at the equator in the prophase and smallest in the telophase. " 



Mid-body. — When the new cell-wall is formed the spindle is constricted in the 

 middle and a very remarkable mid-body (Zwtschenk'orper) is formed. This mid- 

 body is elliptical in outline, and is surrounded by a dark area from which radiations 

 proceed in all directions; into this dark area the cell-membrane and the two halves 

 of the spindle enter, fig. 60. This mid-bocly is for all the world like a centrosome 

 with its surrounding sphere and aster, and recalls Watase's ('93) comparison of the 

 mid-body to an intercellular centrosome. This apparent resemblance is still further 

 supported by the fact that the mid-body in this case becomes a hollow sphere before 

 it finally disappears, fig. 61, just as the centrosome does. 



The mid-body is surrounded by a darkly staining substance which resembles the 

 sphere substance. This recalls Moore's ('93) observations on the larval Salamander, 

 where he finds a mass of archoplasm on each side of the mid-body, also Kostanecki's 

 ('92) statement that the mid-body is formed from granules of the sphere (archo- 

 plasm). Kostanecki ('97) has observed a mid-body in Physa consisting of a ring- 

 around the central spindle-fibres, from which radiations proceed. In some cases this 

 ring divides through the middle into two rings. A similar ring is called by Heiden- 

 hain " Zellnabel." Moore and Meves have seen mid-bodies connected with the 

 centrosomes around the nucleus, as is plainly the case in the third cleavage of 

 Crepidula (see fig. 73). 



The cell-membrane adjoining the mid-body is thicker and more protoplasmic 

 than at the periphery, and is in process of formation at this place. The mid-bod}' 

 persists through the whole of the resting period and until the prophase of the next 

 succeeding division when it gradually disappears. As long as it is present there 

 can be no doubt as to protoplasmic continuity between the daughter cells. 



4. Spheres. — Before the nuclear membrane is indented, the centrosomes are 

 surrounded by a clear area consisting of a halo of radiating fibres, figs. 63, 70, 76. 

 This condition may exist even within the mother centrosome (see fig. 70). This 

 clear area is the first appearance of what I shall call the sphere (" outer sphere" of 

 Lillie, "couche corticale " of Van. der Stricht). When the nuclear membrane is 

 dissolved at the poles substances escape from the nucleus into this area surround- 

 ing the centrosomes. At the same time h}-aloplasm from the cell body is probably 

 drawn in through the astral rays into the same area. There is thus a commingling 

 of hyaloplasm and chromatic nuclear sap which constitutes the interfilar substance 

 of the aster. There is at this stage no clearly marked sphere, since the central area 

 of the aster is in no way delimited from the surrounding radiations. 



In middle stages of mitosis it is difficult, even in thoroughly clestained speci- 

 mens, to trace the polar rays and spindle-fibres through the interfilar substance to 

 the centrosome. In the anaphase the interfilar substance of spindle and aster 



