* 1909] COULTER—EVOLUTION AMONG GYMNOSPERMS 93 
gymnosperms would not materially change this situation. The 
general sequence referred to in the development of the gametophyte 
is as follows: free nuclear division, usually accompanied by vacuola- 
tion which results at some stage in parietal placing; the formation 
of walls, resulting in a parietal tissue; the centripetal growth of this 
tissue until it reaches the center of the embryo sac; and the final 
growth of the gametophyte until it reaches its mature size. In 
certain cases there may be no parietal placing, the free nuclei remain- 
ing distributed throughout the embryo sac; and therefore there is no 
centripetal growth, but general wall-formation throughout the sac. 
The details of the formation of permanent endosperm tissue from the 
primary walled cells are variable and perhaps very important from 
the evolutionary point of view, but the range of forms from which 
these details have been obtained is far too small to make them of 
present service in this connection. 
The tendency which runs through gymnosperms as a whole, and 
which reaches its extreme expression among angiosperms, is to 
mature the eggs earlier and earlier in the ontogeny of the game- 
tophyte. In the most primitive condition of the gametophyte, the 
eggs do not appear until the endosperm is nearly full grown; and 
other gametophytes can be selected and arranged in a series showing 
the gradual slipping-back of the egg in the ontogeny of the game- 
tophyte, until in such a form as Torreya the archegonium initial is 
differentiated as soon as wall-formation has taken place. A conspicu- 
ous illustration of the inequality of response to such a general tendency 
among related forms is furnished by Torreya and Cephalotaxus, 
the archegonia not appearing in the latter genus until the gameto- 
phyte is well grown. The next stage is illustrated by the situation 
in Tumboa, in which eggs are matured before wall-formation is com- 
plete, resulting in the elimination of archegonia. The extreme stage 
in this progressive series of changes among gymnosperms is illus- 
trated by Gnetum, in which eggs are matured at the stage of free 
nuclear division, the most embryonic stage of the female gametophyte. 
So far as the living forms are concerned, the Cycadales and 
Ginkgoales show little, if any, response to this tendency; and there- 
fore possess the most primitive type of female gametophyte among 
q living gymnosperms. Among the Coniferales, on the other hand, 
