CELL FORMATION 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 

 unused 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, c', 

 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 (H). After their forma- 

 tion the cells go on increasing in numbers by simple fission 



Fig. 31. Developmpnt of the cpores of Aopidium 

 JUix-mas. I, the spore-mother-cell, with nucleus ; 

 II, the nucleus absorbed ; ///. two nuclei, and the 

 division of the protoplasm into two portions; IV, 

 four nuclei ; V. division of the protoplasm into four 

 portions ; VI, VII, VIII, rounding up of the young 

 s))ores during the secretion of their cell-walls ; IX, 

 mature spore, with thick and sculptured exospore 

 (epispore). X 550. After Sachs. 



(a) Sachs f makes a strong distinction between the cases of internal 

 cell-formation where, on the one hand, a part only, and, on the other, 



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

 endosperm cells in Duchartre's " Elements de Botanique," pp. 37-89 ; 

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



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

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

 Division of the Mother-Cell. 



