128 ANIMAL PHYSIOLOGY. 
Weismann, in fact, denies that peculiarities acquired dur- 
ing the lifetime of the adult are passed on to offspring. This 
writer believes that we must seek in Ameeba, as the ancestral 
representative of the ovum, for the clew to the laws of heredity. 
The Amceba must divide or cease to exist as a group form— 
hence the segmentation of the ovum; this is but the inherited 
tendency to divide. What the individual becomes is determined 
entirely by the ovum, the whole of which does not develop into 
the new being, but a part is laid aside in reserve as the future 
ovum, Any variations that show themselves in future indi- 
viduals are such as arise from the variations of the ovum itself. 
According to this writer, it is as natural for the offspring to 
resemble the parent (heredity) in the higher groups of animals 
as that one Amceba should resemble another, and for the same 
reason. 
‘Weismann has also attempted to explain the necessity and 
the significance of the extrusion of polar globules. The first 
polar globule is expelled from all ova, even those that can de- 
velop independent of a male cell (parthenogenetic). This rep- 
resents that part of the original ovum which determines its 
peculiarities of form, etc. (ovogenetic idioplasm); while the 
second polar globule is one half of the nucleus of the mature 
ovum ready to enter upon development, if fertilized. When 
the latter takes place, it is joined by the corresponding nuclear 
substance of the male cell to form the segmentation nucleus. 
It is this substance (germ-plasma) which determines exactly 
what line of development, to the minutest details, the ovum 
shall follow. In the course of time the nucleus would thus 
come to represent many generations of united plasmas. There 
must be a limit to this, from the physical necessities of the case ; 
hence the expulsion of a second polar globule, which also is a 
provision against parthenogenesis, for in some cases the plasma 
of the nucleus has the power, without the accession of any 
male plasma, to segment and develop the mature animal. But 
in any case there is a great advantage in the union of the two 
plasmas with their diverse experiences; hence sexual repro- 
duction, though the most costly apparently, is in reality the 
most economical for Nature in the end, for higher results are 
reached, and it seems, in fact, that this lies at the very foun- 
dation of organic progress. 
The theory of Brooks may be regarded as eclectic, being a 
combination of that of Weismann and Darwin more particu- 
larly, with entirely new additions by himself. 
