288 GERM-CELL CYCLE IN ANIMALS 



teristics in the cytoplasm, just as the chromosomes are 

 the bearers of hereditary characteristics in the 

 nucleus, still there are many objections to this view, 

 such as the fact that part or all of the plastosomes 

 may be cast out of the spermatid (e.g., in the opos- 

 sum, Jordan, 1911 ; and in Peripatus, Montgomery, 

 1912). It is obvious from the foregoing account 

 that there are a number of opposing views regarding 

 the origin, nature, and role of the various cytoplasmic 

 inclusions which have been considered mitochondria. 

 Are they constant, necessary constituents of the 

 living protoplasm, or are they inactive lifeless bodies 

 which may be included under the term metaplasm ? 

 If they constitute a part of the living protoplasm, 

 do they form the skeleton of the cell, do they take 

 part in the metabolic activities of the cytoplasm 

 or nucleus, or do they play a role in the process of 

 differentiation, and should they be considered as 

 the hereditary substance of the cytoplasm ? If 

 they are simply metabolic products, are they excretory 

 in nature, or reserve materials set aside for the later 

 use of the cell ? And finally, do they arise from the 

 nucleus, are they strictly cytoplasmic, or do they 

 originate through the interaction of nucleus and 

 cytoplasm ? It is impossible in a short space to 

 give an adequate account of the arguments pro and 

 con, and so we must refer the reader to the compre- 

 hensive reviews mentioned above. The conclusion, 

 however, is perfectly safe that we shall have to await 

 the results of further investigations before we can 

 come to a definite decision. In the meantime we 



