In the following passage, some of the words have been left out. Read the passage carefully and select the correct answer for the given blank out of the four alternatives.
Division of labor is the law in the organic world as in the industrial. Animals ___________ the lowest type, such as the amoeba, do not have separate organs for respiration, digestion, assimilation, elimination, etc. But in the ___________ forms each organ not only has its own specific work, but even within the same organ each part has its own particular function assigned. __________ we have seen that the _________ parts of the neurone probably perform different functions, the cells generating energy and the fibers ______________ it.