1889.] Segincntation of the Ovum. 759 
mation of the polar globules has its long axis coincident with that 
of the ovum, hence the point of exit of the polar globule marks 
one end of the ovic axis. The first amphiaster or spindle is al- 
ways at right angles to the ovic axis. This, however, leaves the 
meridian plane undetermined. Roux, jp, from a series of inter- 
esting experiments on frog's ova, concludes that the plane is fixed 
by the path of the spermatozoon. So far as I know this idea 
was first suggested by Selenka, in 1878, in his paper on " The 
Development of Toxopneustus varicgatusy Compare also Mark, 
32, p. 500. In the frog's egg the path of the male pronu- 
cleus is marked by a line of pigment, as was first described by 
van Bambecke, ^, p. 65, and has been well figured by O. Hert- 
wig, 25a, PI. v., fig. 4. The pigment renders it easy to ascertain 
the position of the male road, even after the first cleavage of the 
ovum. This Roux has done in sectioned ova, and from experi- 
ments and observations reaches this result : The long axis of the 
first segmentation spindle lies in a plane zvhich passes through the 
axis of the ovum and the path of the male pronucleus. If Roux's 
conclusion is confirmed, it will become of fundamental impor- 
tance. Yet there must be other factors which can at least replace 
the male pronucleus in this special role, since the development 
of pathenogenetic ova, in which there is no male pronucleus at 
all, is equally determinate. It is probable that the distribution of 
the protoplasm is the real cause determining the position of the 
nucleus ; thus in oval eggs the spindle lies in the direction of the 
long axis. It is quite probable that if the male pronucleus has 
the effect ascribed to it by Roux, it produces it indirectly by al- 
tering the distribution of the protoplasm within the ovum; that 
such alteration takes place is indicated by the occurrence of the 
male aster. 
That the first cleavage plane is determined by relations existing 
in the unimpregnated ovum has been suggested by O. Schultze 
in consequence of his finding the germinal vesicle lying excen- 
trically in the eggs of the brown frog. Schultze suggests that the 
first plane passes through the ovic axis and the excentric nucleus. 
Roux, Biol. Col., VII., 420, maintains that this suggestion is set 
aside by his ow^n observations cited above. For further discus- 
