UELL FORMATIOJS BY DIVISION. 



41 



about certain points without the previous formation of nu- 

 clei (d, e). In either case firm walls are secreted about the 

 spores while yet in the mother-cell and surrounded by the 

 nnused part of its protoplasm. 



55. — The most striking example of this variety of internal 

 cell-formation is to be found in the • development of the 

 endosperm cells in the embryo sac of Phanerogams. The 

 protoplasm which occupies the cavity of the embryo sac pre- 

 sents here and there points of condensation or concentration, 

 which in a little time become as many nuclei (Fig. 33, A, n, n), 

 each containing a nucleolus. These nuclei are the first in- 

 dications of the form- 

 ing cells. Protoplasm 

 gathers about the nu- 

 clei and forms globu- 

 lar or ovoid masses 

 {A, a, a), which, after 

 acquiring a certain 

 size, secrete a thin 

 wall of cellulose on 

 their surfaces {A,c, d , 

 d). By the continued 

 production of new 

 ■cells within the em- 

 "bryo sac, in this way, 

 they finally become 

 ■crowded together into 

 a, loose tissue, in whose intercellular spaces portions of the 

 ■ unconsumed protoplasm yet remain {B). After their forma- 

 tion the cells go on increasing in numbers by simple fission 

 {B, a, h).* 



{a) Saelis f makes a strong distinction between tlie cases of internal 

 cell-formation wliere, on the one hand, a part only, and, on the otlier. 



Fig. 31.— Development of the ppores of Aspidium 

 Jilix-mae. /, the epore-motlier-cell, with nucleus ; 

 77, the nucleus absorbed ; III, two nuclei, and the 

 division of the protoplasm into two portions ; IV, 

 four nuclei ; F. division of the protoplasm into four 

 poitions ; YI, YII, VIII, rounding up of the young 

 spores durin;^ the secn-tion of their cell-walls ; IX, 

 mature spore, with thick and sculptured exospore 

 (epispore). X 550.— After Sachs. 



* The student is here referred to tlie account of the formation of 

 endosperm cells in Duchartre's "Elements de Botanique," pp. 37-39 ; 

 and also to Hofmeister's "Lehre von der Pflanzenzelle," Section 17. 



f " Lehrbuch," 4te auf. In the English translation of the third edi- 

 tion all cases of fission are included under the Formation of Cells by 

 Division .f the Mother-Cell. 



