The group of the lung-fishes (_Dipneusta_ or _Dipnoi_) is closely connected with the older ganoids. In respect of their whole organisation they are midway between the gill-breathing fishes and the lung-breathing amphibia; they share with the former the shape of the body and limbs, and with the latter the form of the heart and lungs. Of the older dipnoi (_Paladipneusta_) we have now only one specimen, the remarkable Ceratodus of East Australia; its amphiblastic gastrulation has been recently explained by Richard Semon (cf. Chapter XXI). That of the two modern dipneusta, of which _ Protopterus_ is found in Africa and _Lepidosiren_ in America, is not materially different. (Cf. Fig. 51.)
Fig. 49. Ovum-segmentation in the lamprey. Fig. 49—Ovum-segmentation of the lamprey (_Petromyzon fluviatalis_), in four successive stages. The small cells of the upper (animal) hemisphere divide much more quickly than the cells of the lower (vegetal) hemisphere.
Fig.50. Gastrulation of the lamprey. Fig. 50—Gastrulation of the lamprey (_Petromyzon fluviatilis_). A blastula, with wide embryonic cavity (blastocoel, _bl_), _g_ incipient invagination. _B_ depula, with advanced invagination, from the primitive mouth (_g_). _C_ gastrula, with complete primitive gut: the embryonic cavity has almost disappeared in consequence of invagination.
All these amphiblastic vertebrates, _Petromyzon_ and _ Cestracion, Accipenser_ and _Ceratodus,_ and also the salamanders and batrachia, belong to the old, conservative groups of our stem. Their unequal ovum-segmentation and gastrulation have many peculiarities in detail, but can always be reduced with comparative ease to the original cleavage and gastrulation of the lowest vertebrate, the amphioxus; and this is little removed, as we have seen, from the very simple archigastrula of the _Sagitta_ and _Monoxenia_ (see Fig. 29–36). All these and many other classes of animals generally agree in the circumstance that in segmentation their ovum divides into a large number of cells by repeated cleavage. All such ova have been called, after Remak, “whole-cleaving” (_holoblasta_), because their division into cells is complete or total.
Fig.51. Gastrulation of ceratodus. Fig. 51—Gastrulation of ceratodus (from _Semon_). _A_ and _C_ stage with four cells, _B_ and _D_ with sixteen cells. _A_ and _B_ are seen from above, _ C_ and _D_ sideways. _E_ stage with thirty-two cells; _F_ blastula; _G_ gastrula in longitudinal section. _ fh_ segmentation-cavity. _gh_ primitive gut or gastric cavity.