434 Evolution and Adaptation 
polar bodies its male elements, which are again received in 
the fertilization of the egg by the spermatozoon. The same 
idea has also been expressed by others. It has been objected 
to this view that one polar body ought to suffice, and that 
no similar throwing out of part of its substance is found in 
the process of formation of the spermatozoon, which should, 
on the hypothesis, throw out its female elements. It 
would seem, on first thought, that this view might find sup- 
port in the idea expressed above, namely, that in one of 
the polar bodies half of the chromosomes pass out, so that 
there is conceivably a separation of the maternal from the 
paternal. If this were the case also in the spermatozoa, then 
two of each four would be paternal and two maternal. This 
is, however, a very different thing from supposing them to 
be male and female, for it by no means follows, because 
the chromosomes correspond to those of the father or of 
the mother in the sum of their characters, that they are, 
therefore, also male or female in regard to sex. 
It has been pointed out already, that in most partheno- 
genetic eggs only one polar body is extruded. There are, 
it is true, a few apparent exceptions to this rule, but in most 
cases it is certain that only one is extruded. In several 
cases the beginning of the formation of the second matura- 
tion division of the nucleus takes place, but after the chro- 
mosomes have divided they come together again in the 
nucleus. If each polar body be interpreted as equivalent 
to a spermatozoon, then this result is rather a process of 
self-fertilization than true parthenogenesis. It is, neverthe- 
less, true that in some cases development seems to go on 
. after both polar bodies have been extruded. Moreover, it 
has been found possible to cause the eggs of the sea-urchin 
to begin their development by artificial solutions after they 
have extruded both polar bodies. A single spermatozoon 
may also produce an embryo if it enters a piece of egg-pro- 
toplasm without a nucleus. The last instance is a case of 
