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.