ABSTRACT
I. The cortical reaction at fertilization and its timing in sea-urchin eggs
II. The cortical reaction of the unfertilized egg of Sabellaria alveolata
The swelling of the cortical alveoli is the first step in the fertilization reaction of the egg of Paracentrotus’, later on, these alveoli come into contact with the plasma membrane and burst through it.
No vitelline membrane on the surface of the unfertilized egg was seen. But we were also unable to observe any delamination of the plasma membrane at the beginning of the cortical reaction. Immediately after the bursting of the alveoli, their extruded contents lift a thin membrane, which may be considered as a preformed vitelline membrane of the unfertilized egg and is not, therefore, visible while still applied to the plasma membrane.
The cortical alveoli are entirely filled by a convoluted granular membrane. This material becomes homogeneous after bursting out of the alveoli, and some of it lines the inner surface of the vitelline membrane as well as forming the hyaline layer on the outer surface of the egg.
The plasma membrane of the fertilized egg derives from the sides of the alveoli and from those parts of the primitive plasma membrane of the unfertilized egg which were situated between the alveoli.
Having been able to fertilize all eggs of Paracentrotus within the first 10 sec. of mixing the gametes, we may state that activation of the egg begins during this period when the acrosomal tubule first comes into contact with the plasma membrane of the egg.
This activation immediately provokes a swelling of the cortical alveoli which may already be observed after 10 sec. Thus the cortical reaction has nc latent period and may be considered as a progressive and unitary process, starting immediately after sperm contact. The cortical modifications which may be observed in a dark field or with the polarization microscope are due to the first swelling of the alveoli before they burst.
7. The coelomic egg of Sabellaria is covered by a thick and complex chorion, adjacent to the plasma membrane. The egg cortex bears regular, cylindrical microvilli which run perpendicularly through the inner half of the chorion. Under the plasma membrane lies a layer of alveoli filled with fibro-granular material.
8. Immediately after the first contact of the unfertilized egg with sea-water, the cortical alveoli swell and burst at the surface of the egg, their liquefied contents being shed into a perivitelline space. While the microvilli retract from the chorion, the inter-alveolar walls form long villi crossing the vitelline space from the egg surface to the inner surface of the chorion.
9. The only cortical change brought about by fertilization is a progressive retraction of some of the villi. Those which remain become divided by a system of vesicles and irregular cavities.
10. The cortical reaction of the unfertilized egg of Sabellaria brought about by contact with sea-water is, in the main, identical to the fertilization reaction of the sea-urchin.
11. A general and comparative review of the various kinds of cortical reaction leads to the following conclusions : (a) that a normal monospermic fertilization may exist without any morphological reaction (bivalve molluscs) ; {b} that the formation of a perivitelline space after bursting of cortical alveoli may be considered as a protective adaptive device, whose principle is independent of fertilization {Sabellaria} ; (c) that this adaptive reaction has been incorporated through evolution into the fertilization process, thus allowing an improved mechanism for ensuring monospermy (echinoderms and fishes).
12. Attention is drawn to the peculiar morphology of the villi originating from the interalveolar walls ; the pictures observed in both cases (fertilized eggs of Paracentrotus and unfertilized eggs of Sabellaria} suggest an intense pinnocytic activity.
