The nature and origin of the cortical morphogenetic field in Limnaea

Abstract

The vegetative pole plasm in recently laid egg cells of Limnaea stagnalis is situated somewhat obliquely with respect to the longitudinal axis of the first maturation spindle. In the equatorial zone of the egg there are six subcortical “patches” of cytoplasm staining differently from the surrounding cytoplasm. They are arranged according to an asymmetric pattern. These cytoplasmic differentiations of the oviposited egg arise by ooplasmic segregation during the passage of the egg cell through the female genital duct of the parent. They point to a preexistent mosaic pattern in the cortex, which is at the same time polar, dorsoventral, and asymmetric.

The growing oocytes in the gonad lie with one side flat against the acinus wall. They are surrounded by six inner follicle cells in a characteristic arrangement. The oocyte with its surrounding structures also embodies a pattern which is at the same time polar, dorsoventral, and asymmetric.

The patterns of the egg follicle and of the oviposited egg show a great deal of similarity. It is concluded that the mosaic pattern of the cortex of the fertilized egg cell reflects peculiarities in the mutual positions of elements surrounding the oocyte.