The distribution of special cytoplasmic differentiations of the egg during early cleavage in Limnaea stagnalis

Abstract

Uncleaved eggs of Limnaea stagnalis show a pattern of cytoplasmic differentiations in which there are 6 “subcortical accumulations” (SCA) in the equatorial region. SCA consist of a dense cytoplasmic matrix, containing a special kind of small granules. At certain stages also lens-shaped bodies, consisting of coiled threads, are found in them. At the uncleaved stage the SCA form a regular pattern, which is dorsoventral and nearly symmetrical. Its plane of symmetry coincides with the median plane of the future embryo. Just before first cleavage, the SCA exhibit a latitudinal extension and fusion. They are distributed in a regular way among the 2, then 4, blastomeres. Their positions in the 4 quadrants of the egg exhibit characteristic differences. In each quadrant they show a gradual shift toward the vegetative pole.

In centrifuged eggs, the SCA are indistinguishable immediately after centrifuging. In a few hours, however, they are re-formed in their typical localization, even in cleaving eggs.

After first and second cleavage, the spindle remnants are displaced in a vegetal direction in the interblastomeric wall until they lie immediately beneath the surface in the most vegetative part of this wall.

The SCA probably reflect particulars of the cortical morphogenetic field. As their pattern in the uncleaved egg is identical with the pattern of follicle cells surrounding the growing oocyte in the gonad, these observations lend support to the hypothesis that the cortical field is “imprinted” upon the egg during oogenesis by the surrounding structures. Its pattern shows alterations of a dynamic nature during early cleavage, probably connected with reconstruction processes in the cell membrane at cell divisions.