8 INVERTEBRATE MORPHOLOGY. 



Sucli a combination of cytoplasm and caryoplasm consti- 

 tutes a morphological element capable of carrying on all the 

 functions of life. It is not only a morphological but also a 

 physiological element. It is capable of assimilating the 

 necessary substances and building up protoplasm ; metabol- 

 ism and the consequent evolution of energy goes on in it ; it 

 excretes waste products ; it is contractile and may therefore 

 be capable of motion ; it responds to stimuli of various kinds, 

 or in other words it is irritable ; and, finally, it is capable 

 of reproduction. The question naturally arises, however, 

 whether this combination of the two substances mentioned 

 is essential — whether, that is to say, organisms without nuclei 

 do not exist and manifest all the phenomena of life. At 

 one time the existence of unicellular organisms destitute of 

 a nucleus was recognized, the term cytode being applied to 

 them to distinguish them from nucleated cells. Within re- 

 cent years, however, a growing skepticism has come into 

 existence 'as to the non-nucleate character of these organ- 

 isms, the recent improvements of the microscope and the 

 application of modern staining reagents having revealed the 

 existence of nuclei in many of the forms at one time regarded 

 as typical cytodes. It would perhaps be going too far to 

 state that cytodes do not exist, but the evidence at hand indi- 

 cates that their existence is highly problematical. 



This conclusion is strengthened by the results which have 

 been obtained from the observations of artificially produced 

 cytodes. Some of the larger unicellular organisms have 

 been cut into fragments some of which can be definitely 

 shown to be destitute of nuclear or caryoplasmatic substance. 

 In such cases it is found that the nucleated fragments if 

 placed under proper conditions will regenerate and carry on 

 their existence as before, while the cytode fragments, thouo-h 

 manifesting signs of life for a considerable length of time, 

 will not regenerate and do not possess the power of repro- 

 duction. The nucleus seems to possess a marked regulating 

 or coordinating action upon the cytoplasm, coordinating the 

 anabolic and catabolic activities upon which the continuance 

 of life depends. 



It would be beyond the scope of the present work to enter 



