The previously reported observation that micromere formation after cleavage suppression is not linked with the number of blastomeres present but rather with the time schedule of the fourth cleavage of the normal embryos has been confirmed. A hypothesis is advanced that a rhythmical fluctuation of the sulfhydryl contents of the egg proteins is the clock system, and micromere formation is connected with the fourth SH cycle after fertilization. The hypothesis was tested under 3 conditions: (i) Conditions which stop the nuclear activities but preserve the SH cycle, followed by a release from these conditions. (ii) Conditions which "freeze" both nuclear and cytoplasmic rhythms, and later removal of the conditions. (iii) Conditions which leave nuclear activities intact but prevent the cytoplasmic rhythms, followed by normal culturing. The results came out as anticipated by the hypothesis.
When sea urchin embryos at 2‐cell stage are flattered between agar plates, the direction of cleavage is rotated by 90° in each division in reference to the preceding cleavage and no micromere is formed. But under this condition, micromeres are formed in 2 cases; 1) When the egg axis is parallel to the plane of flattening, 2 micromeres are formed on one side of a square 16‐cell stage. 2) when the egg axis is perpendicular to the plane, 4 micromeres are formed at the center of the square. When put into a groove, a string of 4 cells is formed showing that the spindle direction is further deflected by the groove. In the following 16‐cell stage in the groove, which consists of 2 layers of 8 cells, cases with 2 micromeres on one side and 4 micromeres at the center are still found. If the 2‐cell stage is introduced into a groove after the formation of mitotic apparatus, the spindle direction can no longer be changed and the 4‐cell stage becomes like 4 pancakes stuck in 2 layers, indicating that 2 asters are holding the ends of a spindle in fixed positions.
ABSTRACT In the foregoing paper, Ishizaka (1958) succeeded in demonstrating the presence of a pair of ring-zones in the cortex of sea-urchin eggs which remain absolutely stationary with reference to the co-ordinates outside the egg through successive changes of the form of dividing ova. The purpose of the present paper is to see whether or not a similar situation exists in the case of cells cleaving in a heart shape ; i.e. in cells in which the cleavage furrow of one side appears earlier than that of the opposite side.
1. The proposal made by the Andrews and A. R. Moore that sea urchin blastomeres are bound together by means of protoplasmic strands is rejected and the notion is advocated that sea urchin blastomeres are stuck to the inside surface of the hyaline plasma layer by attachment fibers radiating out from the blastomere surface. This mode of fixation secures definite relative positions among the blastomeres. Evidences are put forward indicating the above relation.2. Possible sources of errors in the spinning theory of Andrews and Moore are analyzed.3. Points concerning the movement of the cell surface during cleavage and the pattern of cleavage are considered.
On montre que deux mecanismes interviennent dans la division cellulaire de l'œuf d'oursin, dans un processus de clivage mais ils sont actives successivement. Il est possible de determiner le moment exact du passage du mecanisme «astral» au mecanisme constricteur
