. Journal of morphology. anation will suffice. Let us lookfor a moment at the cleavages of X : The first position of thespindle is on its left side ; the second position on the right side(Figs. 25 and 27) ; the third in themiddle line towards the apical pole(Figs. 33-35); the fourth in themiddle line towards the vegetativepole (Fig. 39). (See accompany-ing diagram.) In none of thesecases does the spindle occupy morethan a fraction of the diameter ofthe blastomere in question. Thenucleus has been wanderingthrough the cytoplasm from oneside to the other, from the frontto the back, stopping at va
. Journal of morphology. anation will suffice. Let us lookfor a moment at the cleavages of X : The first position of thespindle is on its left side ; the second position on the right side(Figs. 25 and 27) ; the third in themiddle line towards the apical pole(Figs. 33-35); the fourth in themiddle line towards the vegetativepole (Fig. 39). (See accompany-ing diagram.) In none of thesecases does the spindle occupy morethan a fraction of the diameter ofthe blastomere in question. Thenucleus has been wanderingthrough the cytoplasm from oneside to the other, from the frontto the back, stopping at variousstations, and giving off a cell ateach one. Finally the nucleus stops in the centre of the celland a perfectly bilateral spindle (the fifth) is formed (Fig. 44).Why docs it stop Is it because its environment haschanged ? If so, the change is such as to elude the closestscrutiny. In fact the cell is a builder which lays one stonehere, another there, each of which is placed with reference tofuture - Diagram of the Cleavages of theFirst Somatoblast X. III. Gastrula to Glochidium. (a) Gastrula to Young Larva. I. Ectodermal Organs. The formation of the shell-gland has already been described. Immediately afterwards the lumen is filled with a transparent, refringent substance, the first indication of the cuticular shell. A rapid evagination of the capacious gland now ensues. At the same time there is a thinning out, which proceeds more rapidly in the centre than at the sides. This is, of course, due to the extension of the area covered by the same cells. When No. I.] THE EMBRYOLOGY OF THE UNIONIDAE. 47 the evagination is completed the shell appears in the form ofa very delicate, transparent, cap-shaped cuticle, covering thewhole of the shell-gland area, and which is derived from theabove-mentioned refringent secretion of the shell-gland. As has often been noted, the shell is not at first bivalve;nor is it so when the gland has evaginated, and the dorsal line
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