ABSTRACT
The nucleolus in the nucleus of an Echinus oocyte always orientates itself gravitationally on the floor of the nucleus. When the oocyte is disturbed the nucleolus falls through the fluid contents of the nucleus with an average velocity of 0·4 μ per sec.
Gravity has no direct action on the direction of the cleavage planes in Echinus eggs, but it orientates the whole egg within the fertilisation membrane.
During the first cleavage the mitotic axis can lie in any position in respect to gravity, but if its position deviates appreciably from the horizontal then (as soon as the cell elongates by cleavage) the whole egg moves so as to bring its centre of gravity into equilibrium with gravity and with the frictional forces acting between the egg and the fertilisation membrane.
During the second cleavage the mitotic axis must lie in a plane parallel to the first cleavage furrow in conformity with Hertwig’s Law. If its position deviates from the horizontal, then the egg orientates itself to gravity. In this way the second division gives rise to four blastomeres resting as a flat plate on the floor of the fertilisation membrane, independently of whatever position was occupied by the mitotic axis.
The third cleavage is also in accord with Hertwig’s Law and no gravitational disturbances occur.
The direction of each cleavage plane is determined by the resultant of three factors: (a) the forces underlying Hertwig’s law, (b) gravity, (c) friction between the egg and its fertilisation membrane.
