154 THE LIVING CYC.\DS 



the spore coat, a structure which was so necessary in 

 the case of spores which were shed, so long after the 

 shedding habit has been lost and the necessity for pro- 

 tection has ceased to exist, is not easy to explain. In 

 other groups of gymnospemis the gradual reduction and 

 disappearance of the spore coat can be traced, and con- 

 sequently it seems to be, not a necessary structure, but 

 one which has been retained by heredity long after it has 

 ceased to perform its original function. 



The germination of the megaspore throughout the 

 group begins with a period of free nuclear division which 

 is more or less prolonged, according to the size of the 

 ovule. The formation of the cell walls and the later 

 development of the female gametophyte are very uni- 

 form in all the genera except Microcycas, in which the 

 cells often have more than one nucleus. The general 

 character of the mature gametophyte is quite uniform, 

 a firm, white, ovoid body with an abrupt depression, 

 the archegonial chamber, at the top. The number of 

 archegonia varies, within narrow limits, ranging from 

 three to ten, except in Microcycas, which often has more 

 than a hundred, not always grouped at the top, but 

 often scattered over the whole surface. The develop- 

 ment of the individual archegonium is quite uniform, 

 there being two neck cells, a ventral-canal nucleus, and 

 an egg. The failure to form a wall between the ventral- 

 canal nucleus and the egg nucleus is an advanced feature, 

 so that in this particular the cycads have advanced farther 

 than the pines, lirs, and spruces, which always form a 

 wall at this point, as did their very remote fern ancestors. 



The free nuclear stage, with the subsequent forma- 

 tion of walls, is an interesting phase in the evolution of 



