Fig. 4.—Ovum of Asterias glacialis, shewing the clear spaces in the place of the germinal vesicle. Fresh preparation (copied from Fol).
At a slightly later stage in the place of the original germinal vesicle there may be observed in the fresh ovum two clear spaces (fig. 4), one ovoid and nearer the surface, and the second more irregular in form and situated rather deeper in the vitellus. By treatment with reagents the first clear space is found to be formed of a spindle with two terminal suns on the lower side of which is a somewhat irregular body (Fig. 5). The second clear space by the same treatment is shewn to contain a round body. Fol concludes that the spindle is formed out of part of the germinal vesicle and not of the germinal spot, while he sees in the round body present in the lower of the two clear spaces the metamorphosed germinal spot. He will not, however, assert that no fragment of the germinal spot enters into the formation of the spindle. It may be observed that Fol is here obliged to fill up (so far at least as his present preliminary account enables me to determine) a lacuna in his observations in a hypothetical manner, and O. Hertwig's (13) most recent observations on the ovum of the same or an allied species of Asterias tend to throw some doubt upon Fol's interpretations.
Fig. 5.—Ovum of Asterias glacialis, at the same stage as Fig. 4, treated with picric acid (copied from Fol).
The following is Hertwig's account of the changes in the germinal vesicle. A quarter of an hour after the egg is laid the protoplasm on the side of the germinal vesicle towards the surface of the egg develops a prominence which presses inwards the wall of the vesicle. At the same time the germinal spot develops a large vacuole, in the interior of which is a body consisting of nuclear substance, and formed of a firmer and more refractive material than the remainder of the germinal spot. In the above-mentioned prominence towards the germinal vesicle, first one sun is formed by radial striæ of protoplasm, and then a second makes its appearance, while in the living ovum the germinal spot appears to have vanished, the outline of the germinal vesicle to have become indistinct, and its contents to have mingled with the surrounding protoplasm. Treatment with reagents demonstrates that in the process of disappearance of the germinal spot the nuclear mass in the vacuole forms a rod-like body, the free end of which is situated between the two suns which occupy the prominence of the germinal vesicle. At a slightly later period granules may be seen at the end of the rod and finally the rod itself vanishes. After these changes there may be demonstrated by the aid of reagents a spindle between the two suns, which Hertwig believes to grow in size as the last remnants of the germinal spot gradually vanish, and he maintains, as before mentioned, that the spindle is formed at the expense of the germinal spot. Without following Hertwig so far as this[364] it may be permitted to suggest that his observations tend to shew that the body noticed by Fol in the median line, on the inner side of his spindle, is in reality a remnant of the germinal spot and not, as Fol supposes, part of the germinal vesicle. Considering how conflicting is the evidence before us it seems necessary to leave open for the present the question as to what parts of the germinal vesicle are concerned in forming the first spindle.
Fig. 6.—Portion of the ovum of Asterias glacialis, shewing the spindle formed from the metamorphosed germinal vesicle projecting into a protoplasmic prominence of the surface of the egg. Picric acid preparation (copied from Fol).
Fig. 7.—Portion of the ovum of Asterias glacialis at the moment of the detachment of the first polar body and the withdrawal of the remaining part of the spindle within the ovum. Picric acid preparation (copied from Fol).