26 KARYOKINESIS. 



perfectly centered. Nevertheless they are the only structures in the egg at this 

 stage which at all resemble centrosomes, and I believe, though I cannot positively 

 affirm it, that they become the centrosomes of the first cleavage spindle. In a 

 slightly more advanced stage, figs. 48, 52, 53, unmistakable centrosomes are present ; 

 they are no larger than the granules of the preceding stage, but the radiations are 

 larger and more numerous, and they proceed in all directions from them. Those 

 radiating fibres which are directed toward the germ nuclei come into contact with 

 the nuclear membrane, which becomes infolded at this point, and at the same time 

 a darkly staining, homogeneous fluid escapes from the nucleus thus forming a cone 

 or half spindle, the base of which is applied to the nucleus, while the apex reaches 

 to and surrounds the centrosome. 



As soon as the undoubted centrosomes appear the fused egg and sperm spheres 

 lose their boundaries, and their granules are either dissolved, or are scattered 

 through the cytoplasm, figs. 48, 52, 53. The cleavage centrosomes are from the 

 first independent of each other, and not until a later stage (figs. 54 and 55), is 

 there any trace of a " central spindle " between them ; these fibres grow out from 

 each centrosome until they meet and fuse, just as MacFarland ('97) has observed 

 in the first cleavage of Pleurophylidia. 



In view of the controversy as to the origin of the cleavage centrosomes in 

 different animals, it is important to know what relation these centrosomes bear to 

 the egg and sperm spheres of Crepidnla. Unfortunately no conclusive answer can 

 be given to this question since the centrosomes do not appear until after the spheres 

 have fused. 1 There are certain evidences, however, which point to the conclusion 

 that each sphere gives rise to one of these centrosomes. The evidences are the 

 following : — (1-) in fig. 45 a number of yolk spherules lie between the egg and sperm 

 spheres which are here entirely separate; in figs. 46 and 47, the principal mass 

 of yolk within the fused spheres probably marks the line of fusion between the 

 two spheres; in fig. 47 a centrosome lies on each side of the principal aggregation 

 of these yolk spherules, and therefore it is probable that one centrosome has arisen 

 from that part of the fused sphere which was the sperm sphere, and the other from 

 the half which was the egg sphere ; (2) until the time of fusion each sphere is 

 closely connected with, in fact partially surrounds, its own nucleus. Even after the 

 fusion it can be seen, fig. 46, that a denser portion of the fused sphere is connected 

 with each of the germ nuclei. Now, if the centrosomes arose, one from the egg 



1 Since this was written more recent work on this subject has shown conclusively that centrosomes 

 and spindles may arise separately in connection with each germ nucleus. If the recently fertilized eggs 

 of C. plana are put into a 1 per cent, solution of sodium chloride in normal sea water for 4 hours, a 

 perfect karyokinetic spindle, though about one-half the size of the usual cleavage spindle, appears in 

 connection with the egg nucleus, although the latter may be separated from the sperm nucleus by 

 almost the whole diameter of the egg. If the sperm nucleus is small and densely chromatic no spindle 

 is formed in connection with it; if, however, the sperm nucleus has grown until it contains a consider- 

 able quantity of achromatic material a perfect spindle may be formed in connection with it also ; in 

 such cases the two spindles usually lie close to each other and may form a tetraster. This experiment 

 suggests that the contradictory observations of different investigators on different animals may find an 

 explanation in the varying rates of growth of the germ nuclei within the egg or in slight differences of 

 the environment. 



