XIIl] THE EMBRYO-SAC A SPORE 121 



mass of protoplasm in the venter is the oosphere, which 

 after fertilisation becomes the embryo. 



If we review the processes above described in the 

 ovules of Angiosperms and Gymnosperms in the light of 

 what is already known, it is evident that the initiation 

 of the sporogenous cells, of which one gives rise to the 

 embryo-sac, is a process of formation of sporogenous 

 tissue, with the accompanying development of investing 

 cells; and the embryo-sac becomes recognisable as a Spore. 

 The initial difficulty which the student may experience 

 in clearly grasping this important homology is due partly 

 to the terminology, and partly to the fact that this spore 

 (the embryo-sac) does not become free : if it escaped, as 

 does the pollen-grain, he would have less difficulty in 

 seeing the resemblances. But there are plenty of cases 

 of undoubted spores of Cryptogams which do not escape 

 from their sporangium before they germinate, and since 

 all the processes in the young nucellus which lead to 

 the formation and completion of the embryo-sac -e.g. the 

 development of the archespore and tapetum are exactly 

 comparable to the establishment of sporogenous tissue and 

 spores in the anther and elsewhere, we must be prepared 

 to examine the detailed evidence which establishes that 

 the embryo-sac is a spore, comparable to the pollen-grain 

 which is also a spore, as we have seen. And the embryo- 

 sac being a spore, the nucellus in which it is developed 

 is a sporangium, and the processes leading to the develop- 

 ment of the more or less rudimentary tissues inside the 

 spore endosperm, archegonium, and oosphere in Gymno- 

 sperms, and egg-apparatus, antipodal cells, &c., in Angio- 

 sperms are comparable to the processes leading to 

 the development of the much more rudimentary tissues 

 in the germinating pollen-grain. In other words the 

 process is that of the germination of the spore in situ. 



