Abstract
In a previous paper 1 it was shown that the ventralmost surface of the zygote of the salamander, Triturus torosus, is carried into the interior of the egg sometime before gastrulation begins. Vitally stained marks were placed on the lower (vegetal) surface of the zygote and development allowed to proceed for 24 to 30 hours. The marks were subsequently located in the uppermost portion of the blastula floor.
In the present experiments large, clear marks were placed on the ventral (vegetal) surfaces of eggs at successive stages of cleavage. The subsequent location of the stained materials was determined by dissecting the eggs in the mid-blastula stage. Observations were made with a device which gives a ventral or lateral view without the necessity of inverting the egg. 1 The ingression described cannot, therefore, be a result of the movement of the heavier yolk material under the influence of gravity.
The results obtained with 34 eggs are given in Table I. Typical examples are illustrated in Fig. 1. From the table it is apparent that ingression is most extensive if the stain is applied prior to the completion of the first 3 cleavage furrows. Stains made after this stage undergo only a relatively small degree of ingression. And finally, if stains are applied when the egg is a well-developed blastula, very little or no ingression at all is discernible.
This relationship between extent of ingression and stage of cleavage gives us an insight into the mechanism involved. 2 The first 3 cleavages are the most extensive in scope, passing through a greater portion of the egg's substance. They are thus capable of displacing the stained ectoplasm to the greatest extent. Later cleavages are progressively localized to smaller and smaller portions of the egg.
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