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 



