The Development of Phascolosoma. 87 



between the sperm nucleus and its centrosome have been found in 

 the mouse by Sobotta (1895) and in Physa by Kostanecki & 

 WiERZEJSKi (1896). Michaelis (1897) also found in Triton several 

 rays extending- from the centrosome to the nuclear membrane of 

 one of the pronuclei, presumably the sperm nucleus. 



The presence of this deeply-staining- connecting filament in 

 Plmscolosoma indicates, it seems to me, that chemical action is going- 

 on between the sperm centrosome and the nucleus. Whatever may 

 move the aster, we may infer that, in this animal, the aster actively 

 draws the nucleus after it into the centre of the ^^g by virtue of 

 what is probably chemical attraction. 



In forms like Fhascolosoma, in which the centrosome arises and 

 remains in close connection with the male pronucleus, we cannot 

 regard the sperm centrosome as a purely cytoplasmic phenomenon, 

 that is independent of the nucleus except as it acquires a secondary 

 connection with it, as the researches of Morgan, Mead, and Wilson 

 have shown to be the case in certain eggs. It seems rather that a 

 definite substance of active chemical nature is introduced by the 

 spermatozoon into the cytoplasm; and that this substance has a 

 strong affinity for the sperm nucleus, but is attracted still more 

 strongly by the mass of yolk-free protoplasm at the centre of the 

 egg. A chemical stimulus is apparently transmitted by it through 

 the cytoplasm to the spindle of the first polar body, setting into 

 operation the process of karyokinesis. 



The spindle of the first polar body at the beginning of this 

 period is in metaphase (Fig. 3 — 6) with ten chromosomes which 

 have typically the shape of elongated rings or rods, which lie parallel 

 to the length of the spindle. This is the reduced number ; for there 

 are twenty chromosomes in the conjugation nucleus and in the 

 spindle of the first cleavage.^) I have found that this number, ten, 

 is characteristic of the first maturation spindle in both Ph. gouldii 

 and Ph. vulgare, and I am informed by my friend Prof. Francotte 



1) In Ph. vulgare, however, I have uniformly found ten chromosomes 

 in the late cleavage and in the gastrula. Thus Figs. 40a and 40b show 

 ten chromosomes at each end of a spindle in anaphase, which has been 

 cut transversely. They lie in a cell of the somatic plate of ectoderm. 

 Cells in other parts of the embryo contain the same number of chromo- 

 somes, each of which I regard as bivalent. Such chromosomes in Pit. 

 vulgare, however , are not limited , as appears to be the case in certain 

 other forms, to the progenitors of the germ cells. 



