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 Pertpatus, Montgomery, 
1912). It is obvious from the foregoing account 
that there are a number of opposing views regarding 
the origin, nature, and réle 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 réle 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 
