Box. -Vol. I.] CANNON— AVENA FATUA. 343 



the nuclei divide in all parts of the embryo-sac wher- 

 ever they are. With their increase in number they become 

 larger in size, cease dividing, are usually spherical, and 

 become enclosed in rather dense masses of protoplasm, 

 which later become separated by cell walls. The lumen of 

 the embryo-sac begins to fill first around the embryo, where 

 the endosperm cells that closely invest it are long, some- 

 what crescent-shaped, and very dense. 



The nucellus and embryo-sac increase greatly in length 

 after the fertiHzation of the ^gg, the embryo-sac absorbing 

 more and more of the nucellus cells, until little more than a 

 single layer separates it from the inner integument. A 

 change also takes place in the integuments. Before fertili- 

 zation and more especially during the youngest stages of 

 the embryo-sac, the two integuments are quite similar, each 

 being composed of two layers of uniform cells. After 

 fertilization the outer integument is compressed and finally 

 destroyed, and the inner one takes on heavier cell-walls and 

 becomes the outer seed-coat. 



VI. The Embryo. 



The oospore, which is relatively small and enclosed by a 

 delicate membranous wall, increases considerably in size 

 after fertilization and before any division occurs. The 

 cytoplasm is granular and with few vacuoles. An interest- 

 ing variation in the cytoplasmic structure of the cells of 

 very 3^oung embryos was met in several instances. The 

 cytoplasm appeared very coarsely granular and net-Hke. 

 No conspicuous vacuoles were present in such embryos; 

 growth and cell-division as far as it had occured were 

 normal. No embryos beyond the proembryo stage pre- 

 sented this pecuHar appearance of the cytoplasm. 



The first division of the ^gg occurs near the distal end 

 and is transverse, either at right angles to the long axis of 

 the young embryo (proembryo), or at an angle more or 

 less acute to it. I found one abnormal case in the first 

 division of the ^gg, in which the cell-wall was a longitudinal 

 one. Figure 29 shows an egg in course of the first division 



