(1) The general appearance of the thickened edge of the blastoderm becoming continuous with the medullary folds has been used as an argument for the medullary folds being merely the coalesced thickened edges of the blastoderm. Since, however, the medullary folds are merely parts of the medullary plate, and since the medullary plate is continuous with the adjoining epiblast of the embryonic rim, the latter structure must be continuous with the medullary folds however they are formed, and the mere fact of their being so continuous cannot be used as an argument either way. Moreover, were the concrescence theory true, the coalescing edges of the blastoderm might be expected to form an acute angle with each other, which they are far from doing. (2) The medullary groove becomes closed behind earlier than in front, and the closure commences while the embryo is still quite short, and _before the hind end has begun to project over the yolk_. After the medullary canal becomes closed, and is continued behind into the alimentary canal by the neurenteric passage, it is clearly impossible for any further increase in length to take place by concrescence. If therefore His' and Rauber's view is accepted, it will have to be maintained that only a small part of the body is formed by concrescence, while the larger posterior part grows by intussusception. The difficulty involved in this supposition is much increased by the fact that long after the growth by concrescence must have ceased the yolk blastopore still remains open, and the embryo is still attached to the edge of the blastoderm; so that it cannot be maintained that the growth by concrescence has come to an end because the thickened edges of the blastoderm have completely coalesced. The above are arguments derived simply from a consideration of the growth of the embryo; and they prove (1) that the points adduced by His and Rauber are not at all conclusive; (2) that the growth in length of the greater part of the body takes place by the addition of fresh somites behind, as in Chætopods, and it would therefore be extremely surprising that a small middle part of the body should grow in quite a different way. Many minor arguments used by His might be replied to, but it is hardly necessary to do so, and some of them depend upon erroneous views as to the course of development, such as an argument about the notochord, which depends for its validity upon the assumption that the notochord ridge appears at the same time as the medullary plate, while, as a matter of fact, the ridge does not appear till considerably later. In addition to the arguments of the class hitherto used, there may be brought against the His-Rauber view a series of arguments from comparative embryology.