8 



INTKODUCTIOX PEOTOPLASM THE CELL 



LAYER OF THE SKIN. 



The intercellular bridges are very distinct. 

 and eosin. x 1,000. 



Hematein 



Hum, muscle, and connective tissue. Similar differential changes 

 acting upon the endoplasm result in the formation of such struc- 

 tures as the contractile fibrils of muscle cells, the neurofibrils of 



nerve cells, the mu- 

 cin, zymogen and 

 secretory granules 

 of epithelial cells, 

 and the fat of con- 

 nective tissue cells. 

 These develop- 

 mental phenomena 

 are, however, not 

 the only evidence of 

 the vital nature of 

 protoplasm. It pre- 



r^r sents certain other 



phenomena, some 



FIG. 8. GROUP OF EPITHELIAL CELLS FROM THE MALPIGHIAN Q \^hich ma\ T be Qli- 



croscopically dem- 

 onstrated, which 

 are accompanied by 

 characteristic histological changes. The vital properties which 

 thus concern the histologist are motion, secretion, growth, and 

 reproduction. 



Motion. Cell motion is that evidence of excitability which 

 results in change of the cell form or position. Three varieties may 

 be recognized in animal cells. 



(a) Amoeboid Motion. This form of cell motion is evidenced 

 by a change of shape of the cell by which protoplasmic processes, 

 pseudopodia, are ^ 

 pushed out in one 

 or more directions. 

 These processes may 

 then be either with- 

 drawn or they may 

 unite with one an- 

 other, and thus, per- 

 haps, inclose a for- 

 eign particle ; or, 



again, the cell body may flow into the pseudopodium, which is 

 thus increased in size, the cell body becoming correspondingly 



FlG. 9. A LEUCOCYTE FROM HUMAN BLOOD IN ACTIVE 

 AMCEBOID MOTION. 



The figures indicate the successive forms assumed by 

 the cell. Drawings were made at intervals of one minute, 

 x 500. 



