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1905] CHAMBERLAIN—ALTERNATION OF GENERATIONS 139 
nucleus, does not differ very markedly from the free nuclear type 
of gametophyte seen in some species of Gnetum. Four mitoses of 
the simultaneous type give rise to these sixteen nuclei, one of which 
becomes the egg nucleus. Between the reduction of chromosomes 
and the fertilization of the egg there are only four mitoses. In 
many angiosperms, as in Lilium, the interval is still shorter, the 
gametophyte containing only eight nuclei, resulting from three mitoses. 
This is the most reduced female gametophyte yet discovered in plants, 
and it shows only one more mitosis than does the gamete-producing 
generation in animals, which regularly shows two mitoses. 
Up to this point I have made no mention of the tetrad of spores, 
a feature almost universal in plants above the thallophytes. Where 
the tetrad is present, the reduction of chromosomes takes place dur- 
ing the two mitoses by which the four spores are formed from the 
mother-cell. It is only in some heterosporous forms that the tetrad 
is incomplete or fails to appear. In such cases the reduction takes 
place during the first two mitoses in the mother-cells, so that whether 
a tetrad is formed or not, the reduction takes place during the first 
two mitoses in the mother-cell. To suggest that the gametophyte 
of the Lilium type represents a reduced tetrad might only cause 
confusion, and since the question does not affect essentially the 
theory proposed, I shall not discuss it at this time, but shall depend 
for comparison upon forms in which a tetrad is present. With a 
few well-known exceptions, one member of the tetrad develops, 
while the other three are evanescent and have no obvious function. 
It is only rarely that any nuclear division occurs in the three evanes- 
cent spores of the tetrad. In the functional spore of the tetrad three 
mitoses occur, giving rise to eight free nuclei, one of which becomes 
the nucleus of the egg. : 
I believe that this latter condition—the most prevalent one in 
angiosperms— corresponds closely with the egg and three polar 
bodies so characteristic of animals. In my opinion, the egg with 
its three polar bodies constitutes a generation directly comparable 
with the gametophytic generation in plants. The accompanying 
diagram (fig. 7) illustrates the comparison. Animals do not furnish 
any example more directly comparable with the condition shown in 
D of the diagram. In D the cell which we have compared with the 
