From the hypoblast are derived the epithelium of the digestive canal, the epithelium of the trachea, bronchial tubes and air cells, the cylindrical epithelium of the ducts of the liver, pancreas, thyroid body, and other glands of the alimentary canal, as well as the hepatic cells constituting the parenchyma of the liver, developed from the hypoblast cylinders given off around the primary hepatic diverticula. Homologous probably with the hepatic cells, and equally of hypoblastic origin, are the spheroidal 'secreting cells' of the pancreas and other glands. The epithelium of the salivary glands, though these so closely resemble the pancreas, is probably of epiblastic origin, inasmuch as the cavity of the mouth is entirely lined by epiblast. The hypoblast also lines the allantois. To these parts must be added the notochord and subnotochordal rod. From the mesoblast are formed all the remaining parts of the body. The muscles, the bones, the connective tissue and the vessels, both arteries, veins, capillaries and lymphatics with their appropriate epithelium, are entirely formed from the mesoblast. The generative and urinary organs are entirely derived from the mesoblast. It is worthy of notice that the epithelium of the urinary glands, though resembling the hypoblastic epithelium of the alimentary canal, is undoubtedly mesoblastic. From the mesoblast are lastly derived all the muscular, connective tissue, and vascular elements, as well of the alimentary canal and its appendages as of the skin and the tegumentary organs. Just as it is only the epidermic moiety of the latter which is derived from the epiblast, so it is only the epithelium of the former which comes from the hypoblast. _Growth in length of the Vertebrate Embryo._ With reference to the formation and growth in length of the body of the Vertebrate embryo two different views have been put forward, which can be best explained by taking the Elasmobranch embryo as our type. One of these views, generally held by embryologists and adopted in the previous pages, is that the Elasmobranch embryo arises from a differentiation of the edge of the blastoderm; which extends inwards from the edge for some little distance. This differentiation is supposed to contain within itself the rudiments of the whole of the embryo with the exception of the yolk-sack; and the hinder extremity of it, at the edge of the blastoderm, is regarded as corresponding with the hind end of the body of the adult. The growth in length takes place by a process of intussusception, and, till there are formed the full number of mesoblastic somites, it is effected, as in Chætopods, by the continual addition of fresh somites between the last-formed somite and the hind end of the body.