GENERAL BIOLOGY 



o 



are in process of reproduction is there any definite- 

 ness of form, shape, or number to the chromatin 

 aggregates. 



Usually we find only one nucleus in a cell, but 

 sometimes many nuclei occur, scattered through the 

 cytoplasm. In plants the cell- wall is an important 

 part of the cell, since it affords the necessary rigidity 



and strength to 

 \\ the plant, funo- 



tioning in this 

 way much as the 

 skeleton does in 

 animals. But we 

 find that the pres- 

 ence or absence 

 of a cell-wall is 

 conditioned 

 largely if not. en- 

 tirely by such a 

 demand. In tis- 

 sues where there 

 are strains and 

 stresses to be 

 borne, cell-walls develop in response to such stimuli, 

 but where they are not necessary they do not appear, 

 or only partially develop. Such tissues, however, 

 are found to have as many nuclei as if they were 

 cut up by cell-walls into individual cells. They con- 

 sist, as it were, of a mass of cells "run together," and 

 for that reason are called coenocytes or syncytia 

 (singular, syncytium). The presence of the 'nucleus 



m. 





B 



FIG. 6. Sections of the outer epithelium 

 (skin) of a creeping Ctenophore (Caeloplana) : 

 A, a ciliated region in which the cells are pro- 

 vided with locomotor organs in the form of 

 vibratile cilia, a gland-cell at the left ; B, a 

 non-ciliated region of the same epithelium ; 

 the cell-walls are barely indicated (except in 

 the gland cells) ; nuclei are, however, abundant. 



