DEVELOPMENT OF CELLS. 



585 



amoeboid manner, and swim about by the aid of ciliary projections, 

 and it is not until they come to rest that they form a cellulose coat 

 (fig. 591) and acquire the appearance of a completely formed cell. 



Formation by Conjugation. This mode of cell-formation, met 

 with in some Algse and in some Fungi (fig. 503, B, b : fig. 512, A, 

 c, d), consists of the transfer of the protoplasmic contents of one cell 

 into the cavity of an adjoining cell, and in the fusion or blending 

 of the protoplasm of the one with that of the other. The spore so 

 formed is called a zygospore. In place of a numerical increase an 

 actual decrease occurs here, from the blending of two into one. 



Cell-division or Segmentation. This is the common mode of 

 cell-formation in the vegetative system of plants, and occurs also in 

 a slightly modified form in the formation of pollen. In this method 

 of cell-formation the newly-formed cells remain for a time or per- 

 manently in contact with each other, and do not become free or 



Fig. 592. 



Section of the outer layers of the rind of Cereus peruvianus soaked in alcohol: a, cortical 

 cells with contracted protoplasm, some with newly formed septa (e); b, cork cells newly 

 formed by division in the outer cortical cells; c, epidermal cells; d, cuticle. Magn. 

 200 diam. 



detached, at least at first. The process takes place in all growing 

 parts of plants, but in the higher classes these regions are only ac- 

 cessible by dissection ; in the lower, and especially in aquatic 

 plants, we are able to observe the process of cell-division in living 

 organisms in all its details ; and it is in these that the phenomena 



