EVOLUTIONARY TENDENCIES AMONG GYMNOSPERMS 425 



belongs, but does indicate the relative rapidity of the changes. For 

 example, it cannot be claimed that Pinus is ancient because it has 

 male prothallial cells, or that it is modern because its sperms are only 

 male nuclei; the only claim that can be made from these facts is that 

 the simplification of sperms has gone on more rapidly in the Pinus 

 line than the elimination of male prothallial cells. Doubtless the 

 same conclusion applies to all characters used in phylogeny, even 

 to those derived from vascular anatomy. The relative position of 

 any form in a scheme of classification can be determined only by 

 averaging all its characters; and its relative age in a scheme of phy- 

 logeny can be determined only by the sure testimony of history. 



The behavior of the chromatin in the final stages of oogenesis 

 in gymnosperms seems to be unique. At the nuclear division giving 

 rise to ventral and egg nuclei the chromosomes are very small. The 

 ventral nucleus soon disintegrates, but the chromosomes of the egg 

 nucleus form a spirem. A period of maturation follows, for which 

 there is no satisfactory account of the behavior of the chromatin. 

 The coarse reticulum of the egg is not chromatin, for most of it may 

 remain after chromatin again becomes demonstrable. Whatever 

 may be the suggested explanation, it is certain that chromatin has 

 not yet been traced from the telophase of the ventral nucleus mitosis 

 to the spirem of the resting egg nucleus with any such certainty as 

 has been done among pteridophytes and angiosperms. 



Fertilization has been studied more thoroughly in Pinus than in 

 any other gymnosperm, and perhaps it represents the general situation. 

 In this case the discharge of the pollen tube injects all of its contents 

 into the cytoplasm of the egg. One of the male nuclei comes into 

 contact with the female nucleus and the nuclear membranes at the 

 surface of contact break down, so that the chromatin of the two nuclei 

 becomes surrounded by the membrane of the egg nucleus. A spirem 

 is formed from the chromatin network of each of the sex nuclei, and 

 each spirem segments into 12 chromosomes. In the fusion nucleus, 

 therefore, there are 12 male and 12 female chromosomes, which do 

 not fuse, but which become so mixed that they cannot be distinguished. 

 Each chromosome sphts and finally 24 chromosomes pass to each pole 

 to form the first two nuclei of the sporophyte generation. In this 

 so-called act of fertilization, therefore, there is no blending of the 



