CELL FORM AXIOM BY DIVISION. 



41 



about certain points without tlie previous formation of nu- 

 clei {d, e). In eitlier 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 {B). After their forma- 

 tion the cells go on increasing in numbers by simple fission 

 {B, a, b).* 



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

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



Fig. 31.— Development of the ppores of Aspifiivm 

 Jtlix-maa. I, the epoie-mother-cell, with niideus; 

 //, the nucleus aliyorbed ; ///, two nuclei, and the 

 division of the prntoplapm into two portions; IV, 

 four nuclei ; V. divieinn of the protoplapm info four 

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

 spores during the secrt'tion of their cell-walls ; Ijt, 

 mature spore, with thick and sculptured exospore 

 (epispore). X 550. — After Sachs. 



* The student is here referred to the 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. 



I '* 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 of the Mother-Cell. 



