KARYOKINESIS. 39 



position at the poles of the nuclei, the latter frequently put out one or more short 

 blunt processes, text figs. VII, VIII, XII. These processes contain chromatin and 

 sometimes dark masses which look like nucleoli. Unlike the nuclear processes of 

 the anaphase, described above, these are usually found on the side of the nucleus 

 away from the centrosome and nearest the mid-body. It is probable that they are 

 withdrawn into the nucleus before the nuclear membrane is dissolved. Their sig- 

 nificance is unknown. 



This completes the account of the cycle of changes which the nucleus undergoes 

 from one prophase of the cleavage to the next. With the exception of certain minor 

 details, as has been pointed out, each cleavage is like every other in the matter 

 of these nuclear changes. Apart from the equal division and distribution of the 

 chromosomes in each mitosis, the most obvious and striking fact in this nuclear cycle 

 is the escape of so large a part of the nuclear constituents into the cell body during 

 mitosis and the reabsorption of a part of these by the daughter nuclei. 



2. Centrosomes and Central Spindles. — a. Centrosomes. — The origin of the 

 centrosomes for the first cleavage has already been described in detail (pp. 

 25-30). These centrosomes are at first minute granules, quite independent of each 

 other. A few fibres are inserted in them and radiate for a short distance into the 

 cytoplasm. Some of these fibres grow toward the nucleus and form a cone or half 

 spindle (figs. 48, 53), while others grow between the two centrosomes and unite them, 

 thus forming a " central spindle " in the manner observed by Hermann ('91), Driiner 

 ('94) and MacFarland ('97). From the time the central spindle appears, the history 

 of the centrosomes of the first cleavage is almost identically like that in the other 

 cleavages so that the following description, unless otherwise specified, applies to any 

 and all of the cleavages. 



The minute centrosomes of the prophase (figs. 52, 53, 54, 63, 70, text fig. IV, a 

 and b) become much larger in the metaphase (figs. 57, 65, 72, 76, text fig. IV, c 

 and d) and stain less deeply at the center. In the anaphase (figs. 58, 59, 66) the 

 centrosomes continue to enlarge, the periphery alone staining with haematoxylin 

 while the central area takes the plasma stain. Finally in the late anaphase and 

 in the telophase the centrosomes become relatively enormous spheres (figs. 60, 67, 

 73, text fig. IV, e and/), frequently 6 to 8 (i in diameter. The peripheral layer or 

 centrosomal membrane grows thinner and thinner until it reaches such a. degree of 

 tenuity as to be scarcely visible, ultimately breaking up into granules (figs. 68, 69, 

 73, 74). In all these respects the metamorphoses of the centrosomes throughout 

 the cleavage are the same as in the maturation divisions. 



The central area of the enlarged centrosome is at first apparently homogeneous 

 (figs. 58, 66), but gradually minute granules begin to appear within it and then 

 extremely delicate threads connecting them into a reticulum (figs. 59, 60, 66, 67, 

 73, text fig. IV). Sometimes one sees, as in figs. 60, 61, 62, one or two granules 

 within the centrosome which are slightly larger than the others; but during the 

 telophase all of these granules are extremely minute and stain very faintly with 

 plasma stains. Gradually they grow larger until they fill the entire centrosome and 



