1. The cells of early amphibian embryos contain ellipsoidal yolk granules. When a granule lies within the mitotic spindle, it is orientated with its long axis parallel to the direction ofthe spindle fibres. Yolk granules lying in the cytoplasm of cells are not specially orientated unless they lie in narrow spaces between cell surfaces; this is true in cells of the moving invagination streams, in the flask cells of the young blastopore, and in elongated cells of the neural groove and lens.

2. The breaking strain of cell surfaces was tested by bringing the cells into surfaces of known strength and observing whether they withstood the applied tension. The evidence indicates:

(a) That the breaking strain of cells from gastrula and tail-bud embryos is some tens of dynes per centimetre.

(b) That the breaking strain is not the same in all cells, being highest in neural tissue and lowest in endoderm during the stages mentioned.

(c) That the breaking strain gradually increases with age.

(d) That in the neural groove stage the strength of the outer concave surface of the groove is greater than that of the inner convex surface.

3. It is suggested that changes in surface tensions may be of importance in bringing about morphogenesis. The possibility is envisaged that the changes in tension may be connected with orientated fibrizations of the surface proteins.

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