In the Tubularidæ and in Hydra an abbreviated development leads no doubt to the absence of a _free_ planula stage, and the absence of a larval form amongst the Ctenophora may, as has already been stated, be probably explained in the same way. The Coelenterata of all the Metazoa are characterized by the greatest simplicity in the arrangement of their germinal layers; and for this reason very considerable interest attaches to the mode of formation of the layers amongst them. Two germinal layers are constantly found, which correspond _in a general way_ to the epiblast and hypoblast. It might have been anticipated that a certain amount of uniformity would have existed in the mode of formation of the layers. This however is not the case. In perhaps the majority of forms they become differentiated by a process of delamination, but in a not inconsiderable minority the two layers owe their origin to an invagination. Delamination is constant (with the doubtful exception of some Tubularidæ) amongst the Hydromedusæ and Siphonophora. It is perhaps in the main characteristic of the Actinozoa. Invagination by embole takes place, so far as is known, constantly amongst the Acraspeda and frequently amongst the Actinozoa; and an epibolic invagination is characteristic of the Ctenophora. If confidence is to be placed in the recorded observations on which this summary is founded, and there is no reason why in a general way it should not be so placed, the conclusion is inevitable that of the above modes of development the one must be primitive and the other a derivative from it, for, if this conclusion be not accepted, the absolutely inadmissible hypothesis of a double origin for the Coelenterata would have to be adopted. Two questions arise from these considerations:-- (1) Which is the primitive, delamination or invagination? (2) How is the one of these to be derived from the other? There is a great deal to be said in favour of both delamination and invagination; but it will be convenient to defer all discussion of the question to the general chapter on the formation of the layers throughout the animal kingdom. The hypoblast cells are often filled with yolk material, and secondary modifications are thus produced in the development. The most important examples of such modifications are found in the Siphonophora and Ctenophora.