a 



14 KARYOKINESIS. 



Griffin points out that the division of the tetrad in Thalassema and ZirphcBa is 

 unlike that in the Copepods, in that in the former each spherule of the tetrad 

 becomes an arm of a cross and that these arms then split longitudinally, whereas in 

 the Copepod type two entire spherules sejjarate from the other two. The former he 

 calls a " spurious tetrad " (cross form), the latter a " tetrad of the Copepod type." 

 In Crepidu I a, ivisi as in Thalassema and ZirpJusa, the tetrads are of the "spurious'' 

 type in the first maturation, whereas in the second maturation we have chromo- 

 somes which, in every respect, resemble tetrads of the Copepod type. 



In the late anaphase of the second maturation the chromosomes which remain 

 in the egg become vesicular and fuse together to form a few vesicles with large 

 granules of chromatin on their walls, Plate II, figs. -34-35. Finally all of these 

 vesicles fuse into one, as is shown in fig. 36, et seq. 



2. Centrosomes and Central Spindles. — The earliest stage at which centro- 

 somes have been seen was in an egg from the oviduct, not yet fertilized, fig. 2. In 

 this egg the centrosomes are ali-eady present as two minute bodies, in contact with 

 the nucleus and without any apparent central spindle or polar radiations. In fact, 

 because of the absence of these radiations it is impossible to be certain that the two 

 granules shown in fig. 2 are really centrosomes. In other eggs from the oviduct, 

 figs. 3-7, into which a spei-matozoon has penetrated, the centrosomes are larger, a 

 central spindle is present and polar fibres are abundant. I have been unable to 

 determine whether this central spindle arises as a centrodesmus (Heidenhain) or 

 whether its fibres grow out indejaendently from the two centrosomes and afterwards 

 unite to form the spindle. In general it may be said, that the formation of the 

 mitotic figure usually begins with the entrance of a spermatozoon into the egg. 



In the prophase of the first maturation the centrosomes are minute densely 

 staining points ; they grow larger as mitosis advances, and in the stages immediately 

 preceding the metaphase, figs. 8, Sa-, 11, and text fig. Ill, they are more or less 

 irregular in shape, and when deeplj^ stained and strongly destained with the iron- 

 alum-haematoxylin of Heidenhain, they may be seen to contain a central clear area. 

 Around this clear area the dense walls of the centrosome are thickened in places and 

 may, perhaps, represent large granules in contact with one another, as Lillie has 

 found to be the case in Unio. In the pi'ophase of the second maturation, the cen- 

 trosomes are so small that I have found it impossible to make out their structure 

 with certainty, but they are in many cases slightly irregular in form (cf. figs. 27-31), 

 from which I conclude that their structure is the same as in the prophase of the 

 first maturation. 



In the metaphase of the first maturation (fig. 12), the centrosomes are spherical 

 bodies about 1 fi in diameter. Thej^ contain a central area which stains faintly, 

 around which is a thick, dense zone which corresponds to the irregular or granular 

 zone of the prophase; in the metaphase, however, this zone is perfectly regular and 

 gives no indication of being composed of granules as in the preceding stage. 



In the anaphase of the first maturation the centrosomes become large hollow 

 spheres, the peripheries of which stain deeply while their central areas remain 



