1. The structure of the nucleus of the young A. proteus has been described.

2. The development of the nucleus of A. proteus has been traced out.

3. Light has been thrown on some points of difficulty in the interpretation of the structures in the nucleus of the adult A. proteus, both in the resting and in the dividing stages.

4. Comparisons with other forms have been made, and some criticisms have been answered.

5. A method of obtaining stained preparations of fission of the cytoplasm has been given.

6. The investigation has failed to reveal any trace of syngamy in the life-history of A. proteus.

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