A STATISTICAL STUDY OF MITOSIS AND AMITOSIS IN THE ENTODERM OF FASCIOLARIA TULIPA, VAR. DISTANS

1. During the period of cannibalism, the entoderm of Fasciolaria becomes first spindle-shaped, but later as regional differentiation occurs, the cells become cuboidal.

2. The first change can be accounted for by the stretching which the larvæ undergo; the second change is explained by a fourfold increase in the number of cells found in transverse sections through the middle of the digestive tract.

3. During this period of cell increase there was found a maximum of one mitotic division in 1,751 nuclei.

4. During the same period of development, there was found a minimum of 69 amitotic divisions.

5. From this it follows that during the period of most active cell multiplication more than I per cent. of all divisions is mitotic and more than 98 per cent. are amitotic.

6. Since there were found during the pre-cannibal, the cannibal, and the post-cannibal periods, 152 cases of what is interpreted as nuclear division, and since of these 20 were mitotic, it follows that during the entire developmental period considered a little over 13 per cent. of all the divisions were mitotic and a little less than 87 per cent. amitotic.

7. Therefore the conclusion is reached that amitosis plays in this instance an important, if not the chief part in the differentiation of a definitive tissue.

8. Of the two alternatives which might be suggested, one, that unobserved epidemics of mitosis account for the facts, is not only without foundation, but is improbable ; the other, that a fourfold increase in cells can be accounted for on the basis of I mitotic division per 1,751 nuclei involves an absurdity.