Chilostomata and Ctenostomata. As an example of the first type of larvæ, _Alcyonidium mytili_, one of the Ctenostomata, may be conveniently selected for description, as having been more completely worked out by Barrois than perhaps any other form. The segmentation commences in the normal manner by the appearance of two vertical furrows followed by an equatorial furrow, which divide the ovum into eight equal spheres. The stage with eight spheres is followed, according to Barrois, by one with sixteen, formed in a remarkable manner by the simultaneous appearance of two vertical furrows, _both parallel to one_ of the original vertical furrows, so that the segmentation spheres at this stage are arranged in two layers of eight each. In the next stage segmentation takes place along two fresh vertical planes, similar to those of the last stage, but at right angles to them, and therefore parallel to the second of the two primitive vertical furrows. At the close of this stage there are thirty-two cells arranged in two layers of sixteen each; and when viewed from the surface each of these layers presents a regularly symmetrical pattern. Up to the stage with sixteen cells the two poles of the egg, separated by the primitive equatorial plane of segmentation, remain equal, but during the stage with thirty-two cells a peculiar change takes place in the character of the cells at the two poles. At the one pole, which will be spoken of as the oral pole, the four central cells become much larger than the twelve peripheral cells. The stages immediately following are still involved in much obscurity, and have been described very differently by Barrois in his original memoir (No. 298), and in a subsequent note (No. 307)[122]. In the latter he states that the four large cells of the oral face become enclosed by the division and growth of the twelve peripheral cells. They are thus carried into the interior of the ovum; and there divide into a central vitelline mass--the hypoblast--and a peripheral mesoblastic layer. [122] The note (No. 307) refers in the first instance to the changes in the larvæ of the Chilostomata, but the similarity of the larvæ of the Ctenostomata to those of the Chilostomata renders it practically certain that the corrections, in so far as they apply to the one group, apply also to the other.