Io BOTANICAL GAZETTE [JANUARY 
rule of rare occurrence and are distributed among entirely unrelated 
families, on the other hand, the ordinary 8-nucleate type developed 
by 5 divisions of the megaspore mother cell occurs in nearly all 
families of angiosperms from the most primitive to the most 
specialized. This is significant and is hardly to be explained on 
any other hypothesis than that the latter type is primitive. We 
have in the 8-nucleate sac, it seems, a structure of marvelous 
constancy in development and arrangement of parts, evolved in 
the course of long ages, the exceptions to which only strengthen 
the view that it is a primitive type of embryo sac. 
Another reason for regarding anomalous types of embryo sac 
as derived is the variability in the manner of their development. 
This is especially marked in the case of 16-nucleate sacs, where a 
peculiar method of development is found in almost every new case 
discovered. Of course it may be objected that these variations 
are of minor importance. But even if this were true, it certainly 
contrasts strikingly with the remarkable uniformity so very general 
even in minor details of the development of the ordinary angio- 
spermous type, and is a fact to be explained. 
Whether or not we consider the first 4 nuclei produced by 
division of the embryo sac mother cell as megaspores, and this view 
seems reasonable in most cases, is probably of less importance than 
many have believed. The genesis of the angiospermous sporo- 
phyte, aside from the exceptional cases such as those involving 
apogamy and budding, always begins with a perfectly definite 
structure, the embryo sac mother cell. If fertilization is to occur 
later, then, without exception, reduction of chromosomes takes 
place at the first two divisions of this cell. Now, considering 
parthenogenesis as a secondary phenomenon, we see that in the 
development of all normal embryo sacs (that is, those capable of 
being fertilized), whatever their type of mature structure, there is 
this common feature in development. After the reduction divisions 
the embryo sac may develop as the product of one-fourth, one-half, 
or all of the four reduced nuclei derived from the original mother 
cell. The ordinary 8-nucleate type develops as the product of 
one-fourth of these nuclei by three successive divisions; the sac 
of Cypripedium (20) develops as the product of one-half of these 
