FURTHER STUDIES OF EXOGASTRULATION IN THE SEA URCHIN

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Abstract
1. The phenomenon of exogastrulation characterized by inhibition of the gastrulation processes, external differentiation of endoderm, and the excessive proliferation of mesenchyme, has been produced in Arbacia punctulata by exposure of blastula stages to hypotonic sea water, lowering and elevation of temperature, and X-rays. 2. Dilutions of sea water of 45 to 65 per cent give the most numerous cases of the various methods employed. A few occur at 30 to 40 per cent dilutions. 3. Development is typical at 27° C. but at higher temperatures becomes increasingly irregular and abnormal. A temperature of 35° C. kills the cultures or if exposure is for a short time only, inhibits any further differentiation. Shallow exogastrulæ appear at 28° to 31° C. 4. Lowering of temperature has little effect upon gastrulation. Development is fairly typical but retarded at 7° C. At 2° to 4° C. an occasional exogastrula occurs while the stronger individuals eventually gastrulate at this temperature. 5. In the X-ray experiments, dosages of 720 revolutions per minute at 20 cm. distance for 6 to 12 minutes give a few exogastrulæ. 6. Results indicate that stimuli other than chemical ones will inhibit gastrulation at the blastula stage of Arbacia. Great injurious effect on subsequent development is apparent since the exogastruæ fail to differentiate except for apical plate and stomodæum. The exovation is usually shallow. 7. Fusions, ectodermal constrictions, and permanent blastuæ are common even where no exogastrulæ appear. 8. With increase in length of exposure and in strength of stimulus, the more resistant individuals are affected while the weaker ones are killed or fail to show any further development. Various stages of blastulation appear equally sensitive. 9. Examination of the long list of stimuli which will cause exogastrulation would seem to show that any mechanical explanation is unsatisfactory since this phenomenon can be provoked by exposing fertilized eggs, cleavage and blastula stages. It is concluded that some potentiality present in the vegetal pole of the fertilized egg and later segregated in the vegetal cells (presumptive endoderm) of the blastula causes the inturning of cells during gastrulation. This is very sensitive to environmental changes at any stage of development up to the time of gastrulation but varies in susceptibility in different individuals. Accompanying phenomena of cell division and differentiation are also affected according to the stimulus employed and length of exposure.

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