The segmentation is unequal (__vide__ p. 100) and results in the formation of a blastosphere with a large segmentation cavity. The greater part of the circumference of the egg is formed of small uniform spheres, but the lower (with reference to the segmentation) pole is taken up by a single large cell. The small spheres become the epiblast, and the large cell gives rise to hypoblast and mesoblast[108]. [108] The account of the remainder of the development till the larva becomes hatched is taken from Rabl, No. 292. The single large cell next divides into two, and then four, and finally into about ten to fifteen cells. These cells form an especial area of more granular cells than the other cells of the blastosphere. Most of them are nearly of the same size, but two of them (according to Rabl), in contact with each other, but placed on the future right and left sides of the embryo, are considerably larger than the remainder. These two cells soon pass into the cavity of the blastosphere, while at the same time the area of granular cells becomes flattened out, and then becomes involuted as a small sack with a transversely elongated opening, which does not nearly fill up the cavity of the blastosphere. This involuted sack is the archenteron. The two large cells, which lie in immediate contact with what, following Rabl, I shall call the anterior lip of the blastopore, next bud off small cells, which first form a layer covering the walls of the archenteron, but subsequently develop into a network filling up the whole cavity of the primitive blastosphere. The space between these cells is the primitive body cavity. For a long time the two primitive mesoblast cells retain their preponderating size[109]. At the hinder end of the body, and at the end opposite therefore to the two mesoblast cells, are placed three especially large epiblast cells. [109] In this description I follow Rabl's nomenclature. According to his statements the ventral part of the body is the original animal pole--the dorsal the lower pole; the anterior end the mesoblastic side of the opening of invagination. In Anodonta and Unio tumidus there appears at this period a patch of long cilia at the anterior end of the body. These cilia cause a rotation of the embryo and would appear to be the velum. In Unio pictorum they do not appear till much later.