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THE AMERICAN NATURALIST [Vol. XLIV 



other, possibly indicating the future elimination of the 

 smaller nucleus. 



That both the male cell of Thuja and the male nucleus 

 of Pinus are descendants of swimming sperms is not to 

 be doubted. The male cell of Thuja has lost its cilia and, 

 perhaps, is no longer formed within a parent cell, while 

 Pinus has gone further and no longer organizes a defi- 

 nite male cell. In this respect, the Pinus gametophyte 

 is more widely separated from the ancestral form than 

 is the gametophyte of Thuja. 



In some genera, like Torreya and Taxus, the reduc- 

 tion has proceeded in another direction, two male cells 

 being retained, but one of them having become much 

 smaller than the other and having ceased to function, 

 indicating its future elimination. 



It is interesting to note that in Coniferales, with the 

 exception of the Podocarpese, those forms with definitely 

 organized male cells have no prothallial cells in the pollen 

 grains, while those with the free male nuclei have re- 

 tained more or less of the ancestral prothallium; e. g., 

 Pinus has retained the prothallial cells, but no longer 

 organizes a definite male cell, while Thuja lias retained 

 the definitely organized male cell, but has lost the pro- 

 thallial cells. And further, those genera which have 

 retained the definitely organized male cell no longer 

 organize a definite ventral canal cell, having lost the 

 wall between the ventral canal nucleus and that of the 

 egg, a step toward the complete elimination of even 

 a ventral canal nucleus. Whether there are any causal 

 relations among these reductions is not obvious, but 

 it is interesting to note the correlation. If all evo- 

 lutionary lines would only progress at the same pace, 

 or if we could discover causal relations between the 

 lines, it would facilitate the construction of phylogenies. 



Oogenesis 



We have seen that in spermatogenesis the gymno- 

 sperms show a reduction series from the highly differ- 

 entiated motile sperms of cycads to the free male nuclei 



