PROTOPLASM AND LIFE. 



725 



become differentiated off from the remainder, and forms what is known 

 as a nucleus, while the protoplasm forming the extreme outer boundary- 

 differs slightly from the rest, being more transparent, destitute of 

 granules, and apparently somewhat firmer than the interior. We may 

 also notice that at one spot a clear spherical space has made its appear- 

 ance, but that while we watch it has suddenly contracted and vanished, 

 and after a few seconds has begun to dilate so as again to come into 

 view, once more to disappear, then again to return, and all this in 

 regular rhythmical sequence. This little rhythmically pulsating cavity 

 is called the " contractile vacuole." It is of very frequent occurrence 

 among those beings which lie low down in the scale of life. 



We have now before us a being which has arrested the attention 

 of naturalists almost from the commencement of microscopical obser- 

 vation. It is the famous Amoeba, for which ponds and pools and gut- 

 ters on the house-roof have for the last two hundred years been ran- 

 sacked by the microscopist, who has many a time stood in amazement 

 at the undefinable form and Protean changes of this particle of living 

 matter. It is only the science of our own days, however, which has 

 revealed its biological importance, and shown that in this little soft, 

 nucleated particle we have a body whose significance for the morphol- 

 ogy and physiology of living beings can not be over-estimated, for in 

 Amoeba we have the essential characters of a cell, the morphological 

 unit of organization, the physiological source of specialized function. 



The term " cell " has been so Ions' in use that it can not now be 

 displaced from our terminology ; and yet it tends to convey an incor- 

 rect notion, suggesting as it does the idea of a hollow body or vesi- 

 cle, this having been the form under which it was first studied. The 

 cell, however, is essentially a definite mass of protoplasm having a 

 nucleus imbedded in it. It may or may not assume the form of a 

 vesicle ; it may or may not be protected by an enveloping mem- 

 brane ; it may or may not contain a contractile vacuole ; and the 

 nucleus may or may not contain within it one or more minute secon- 

 dary nuclei or " nucleoli." 



Haeckel has done good service to biology in insisting on the ne- 

 cessity of distinguishing such non-nucleated forms as are presented by 

 Protamoeba and the other Monera from the nucleated forms as seen 

 in Amoeba. To the latter he would restrict the word cell, while he 

 would assign that of " cytode " to the former. 



Let us observe our Amoeba a little closer. Like all living beings, 

 it must be nourished. It can not grow as a crystal would grow by 

 accumulating on its surface molecule after molecule of matter. It 

 must feed. It must take into its substance the necessary nutriment ; 

 it must assimilate this nutriment, and convert it into the material of 

 which it is itself composed. 



If we seek, however, for a mouth by which the nutriment can en- 

 ter into its body, or a stomach by which this nutriment can be digest- 



