20 MORPHOLOGY OF SPERMATOPHYTES 



as the information goes, tiie sequence of events seems to be 

 as follows: 



The nucleus of the megaspore divides, and free nuclear 

 division continues until a number of nuclei are formed, all of 

 the divisions being simultaneous, as evidenced by the fact that 

 all the nuclei in the embryo sac show the same karyokinetic 

 phase. Early in these divisions the free nuclei pass to the wall 

 of the sac and become imbedded in a parietal cytoplasmic 

 layer, in which position the subsequent free nuclear divisions 

 are effected. This was the earliest stage of the endosperm ob- 

 served by Ikeno, who traced the subsequent stages in consider- 

 able detail. As the nuclei continue to divide, walls begin to 

 appear, and a parietal tissue is organized which gradually de- 

 velops into and fills up the central cavity of the embryo sac. In 

 this advance of tissue into the central region of the sac Ikeno 

 confirms the statement of Mile. Sokolowa ^" in reference to the 

 development of endosperm in various Conifers, that the endo- 

 sperm cells which are growing into the cavity of the sac have 

 no walls on the inner side. During the progress of endosperm 

 development the embryo sac continues to enlarge, encroaching 

 extensively upon the surrounding nticellar tissiie. Warming re- 

 ports that in the case of Cycas circinalis, if fertilization does 

 not occur, the endosperm continues to grow, protrudes through 

 the micropyle, and being thus exposed to light develops chloro- 

 phyll. 



The development of the archegonium has been traced in 

 great detail by Ikeno ^^ in Cycas re valuta, and in the main con- 

 firms the more general accounts of previous observers. His 

 results are as follows: Soon after pollination (about July 1st in 

 Japan), two to six (mostly three, sometimes eight or more) 

 peripheral cells toward the micropyle become distinguishable 

 from their fellows by their greater size and less deeply staining 

 contents, and are the archegonium initials (Fig. 20, A, a), a 

 condition which lasts but a few days. The second period of de- 

 velopment extends from the appearance of a periclinal wall in 

 the initial cell to the cutting off of the ventral canal cell, and 

 lasts over three months. By means of the periclinal wall the 

 initial cell is divided into an outer primnnj neck cell and an 

 inner central cell (Fig. 20, B). The former divides once by 

 an anticlinal wall, forming a two-celled neck, as in Ginkgo. In 



