FERTILIZATION AND ARTIFICIAL ACTIVATION IN THE EGG OF THE SURF-CLAM, SPISULA SOLIDISSIMA

¹ Department of Zoology, University of Pennsylvania, Philadelphia 4, Penna.

1. The present study is a preliminary survey of fertilization and artificial activation in the egg of the surf-clam, Spisula solidissima (Dillwyn).

2. The structure of the egg, optimal conditions for fertilization, and normal early development of fertilized and artificially activated eggs are described.

3. The results of treatment by various parthenogenetic agents are discussed with particular reference to possible similarities and differences in their mode of action and pattern of response initiated. Among the agents discussed are: ultraviolet irradiation, potassium, sodium, ammonia, osmotic stimuli, heat, cold, urea, and protamine (clupein).

4. The influence of various changes in environmental conditions has been investigated and the results correlated with the function of the cortical region of the cytoplasm during the first four or five minutes immediately following stimulation. Monovalent cations, temperature shock, and stimulation in the cold all increase excitability. Divalent ions, stimulation at slightly elevated temperatures, lowered pH, or the addition of ether, urethane or egg jelly decrease excitability.

5. Sodium-potassium antagonism may be largely responsible for maintenance of the egg in the germinal vesicle stage prior to fertilization.

6. The changes which can be detected in the interior cytoplasm and in the nucleus following activation are discussed with particular reference to changes of shape and volume, and to nuclear breakdown.

7. It is concluded that shape changes are probably caused by an expulsion of water (syneresis) from the cytoplasmic gel when the egg is activated. This expul- sion of water is probably due to an increased gelation caused by a release of calcium from the cortex by activating agents.

8. The response mechanism of the egg can be divided into two stages on the basis of the fact that for 4-5 minutes after activation it is susceptible to inhibition by acid sea water, by lack of calcium, or by dilute ether; but after this time inhibition by these agents is no longer possible.

9. Evidence concerning the direct cause of germinal vesicle breakdown is discussed, and this evidence suggests that a calcium-activated proteolytic or lipolytic enzyme is involved.

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