MORPHOLOGY 



65 



divides simultaneously into as many portions as there are nuclei. In 

 this processwe have an example of multicellular formation. This method 

 of development is especially 

 instructive in the embryo-sac 

 of Phanerogams, a cell, often 

 of remarkable size and rapid 

 growth, in which the future 

 embryo is developed. The 

 nucleus of the embryo -sac 

 divides, the two daughter 

 nuclei again divide, their 

 successors repeat the pro- 

 cess, and so on, until at last 

 thousands of nuclei are often 

 formed. No cell division 

 accompanies these repeated 

 nuclear divisions, but the 

 nuclei lie scattered through- 

 out the peripheral, cyto- 

 plasmic lining of the em- 

 bryo-sac. When the embryo- 

 sac ceases to enlarge, the 

 nuclei surround themselves 

 with connecting strands, 

 which then . radiate from 

 them in all directions (Fig. 



67). Cell-plates make their appearance in these connecting strands, 

 and from them cell walls arise. In this manner the peripheral proto- 

 plasm of the embryo-sac divides, simultaneously, into as many cells 

 as there are nuclei. 



Various intermediate stages between simultaneous,, multicellular formation and 

 successive cell division can often be observed in an embryo -sac. Where the 

 embryo-sac is small and of slow growth, successive cell division takes place, so that 

 multicellular formation may be regarded as but an accelerated form of successive 

 cell division, induced by an extremely rapid increase in the size of the sap cavity. 



Free Cell Formation. — Cells produced by this process differ con- 

 spicuously from those formed by the usual mode of cell division, in 

 that the free nuclear division is followed by the formation of cells which 

 have no contact with each other. This process can be seen in the 

 developing embryo of the Gymnosperms, in Ephedra, for example, and 

 also in the formation of the spores of the Ascomycetes. In the case of 

 Ephedra there first occurs a free division of the nucleus of the fertilised 

 egg ; each daughter nucleus then divides once or twice, so that four 

 or eight nuclei are ultimately produced. A rounded, cytoplasmic 

 mass collects about each nucleus and surrounds itself with a cell wall 



Fig. 67. —Portion of the peripheral protoplasm of the 

 embryo-sac of Reseda odorata, showing the com- 

 mencement of multicellular formation. (X 240.) 



