The larva, after it has left the parental tissues, has an oval form and is transversely divided into two areas (fig. 65 A). One of these areas is formed of the elongated, clear, ciliated cells, with a small amount of pigment near their inner ends (_en._), and the other and larger area of the thirty-two granular cells already mentioned (_ec._). Fifteen or sixteen of these are arranged as a special ring on the border of the clear cells. In the centre of the embryo is a segmentation cavity (_c.s._) which lies between the granular and the clear cells, but is mainly bounded by the vaulted inner surface of the latter. This stage is known as the amphiblastula stage. During the later periods of the amphiblastula stage a cavity appears in the granular cells dividing them into two layers. After the larva has for some time enjoyed a free existence, a remarkable series of changes take place, which result in the invagination of the half of it formed of the clear cells, and form a prelude to the permanent attachment of the larva. The entire process of invagination is completed in about half an hour. The whole embryo first becomes flattened, but especially the ciliated half, which gradually becomes less prominent (fig. 65 B); and still later the cells composing it undergo a true process of invagination. As a result of this invagination the segmentation cavity is obliterated, and the larva assumes a compressed plano-convex form, with a central gastrula cavity, and a blastopore in the middle of the flattened surface. The two layers of the gastrula may now be spoken of as epiblast and hypoblast. The blastopore becomes gradually narrowed by the growth over it of the outer row of granular cells. When it has become very small the attachment of the larva takes place by the flat surface where the blastopore is situated. It is effected by protoplasmic processes of the outer ring of epiblast cells, which, together with the other epiblast cells, now become amoeboid. They become at the same time clearer and permit a view of the interior of the gastrula. Between the epiblast cells and the hypoblast cells which line the gastrula cavity there arises a hyaline structureless layer, which is more closely attached to the epiblast than to the hypoblast, and is probably derived from the former. A view of the gastrula stage after the larva has become fixed is given in fig. 66. [FIG. 66. FIXED GASTRULA STAGE OF SYCANDRA RAPHANUS. (Copied from Schulze.) The figure shews the amoeboid epiblast cells (_ec._) derived from the granular cells of the earlier stage, and the columnar hypoblast cells, lining the gastrula cavity, derived from the ciliated cells of the earlier stage. The larva is fixed by the amoeboid cells on the side on which the blastopore is situated.]