THE CELL 99 



that has no true cell wall is actively amoeboid and 

 appears able to take in food particles through any part 

 of its surface. In more highly specialized protozoa 

 (corticata), such as Paramcecinum, Kerona, and Vorti- 

 cella, there is a well-defined somewhat rigid cell envelope 

 which prevents amoeboid movement and determines 

 that food can be ingested only at a given point in the 

 body surface the oral aperture. 



In certain encysted cells, such as coccidia, the cell wall 

 becomes a dense tough thick structure able to resist 

 drying and other unfavorable conditions, and in the 

 spores of bacteria the resisting power of the cell wall is 

 still more highly developed. 



The ova of tape-worms are not only surrounded by a 

 very tough cell wall, but this in turn by a layer of mucous. 



The mammalian ovum has a highly developed cell 

 wall known to histologists and embryologists as the 

 zona pellucida. It is a thick, hyaline structure, has 

 numerous projecting short filaments, and appears to 

 have many perforations large enough to admit the head 

 of a spermatozoon. 



The highest development of the cell wall is, however, 

 found in the metaphyta or higher plants, where, however, 

 it becomes a product of the cell rather than an integral 

 part of it. Growing plant cells have very thin walls, 

 but so soon as the cells have attained their full size, the 

 cell wall begins to increase in thickness, first by intussus- 

 ception and later by the deposition of new layers upon 

 the originally formed wall. In this process cellulose is 

 the primary product, pectin, lignin, and suberin being 

 secondary products later deposited. The vegetable 

 cell, in the sense of protoplasmic mass, thus comes to 

 lie loosely in a circumscribed space, the walls of which 

 are of its own formation, but to which it is not indis- 

 solubly bound and of which it is not itself a part. 



Thus it appears that the botanist and the zoologist 

 mean somewhat different things when they speak of the 

 cell wall. 



