158 THE LIVING CYCADS 



the genera, but there are differences which enable one 

 to see an evolutionary sequence. 



In all cases the development begins with simul- 

 taneous, free nuclear division; later, walls appear 

 throughout the entire egg or only at the base; and finally 

 the embryo proper, with its suspensor, root, stem, and 

 leaves, is developed from a comparatively small group 

 of cells at the base of the egg. It is an excellent example 

 of what is called meroblastic embryogeny. 



The embryos of homosporous ferns, which have given 

 rise to the heterosporous ferns and, through them, to the 

 early seed plants, have no free nuclear stage in their 

 development. Every nuclear division was followed by 

 the formation of a wall. The free nuclear habit arose, 

 as in the case of the female gametophyte, when the mass 

 of the egg became large in proportion to the size of its 

 nucleus. In all the known ferns, both homosporous 

 and heterosporous, the egg is rather small, and its first 

 division is followed by the formation of a cell wall. It 

 is known that some of the Cycadofihcales had very 

 small seeds and therefore still smaller female gameto- 

 phytes and eggs. In such eggs it is quite possible that 

 walls were formed at every nuclear division, and that 

 the entire egg participated in the formation of the 

 embryo. As the seeds became larger it is known that 

 the gametophytes and eggs also became larger. We 

 beheve that when the egg reached a certain size the 

 mechanism of the dividing nucleus became inadequate 

 to divide the increased mass, so that the two nuclei 

 resulting from the division were left free in the proto- 

 plasm, where they again divided, and so continued to 

 divide until the mass of protoplasm about the individual 



