United States or Niger ? Vote for the TOP Country of the Week !
These important early embryonic processes take place so quickly in the Amphioxus that four or five hours after fecundation, or about midnight, the spherical blastula is completed. The wall is at once gut-wall and body-wall. It is composed of two simple cell-layers, the familiar primary germinal layers.
The freshwater polyp Hydra, like the foregoing illustrations, is one whose structure has already been discussed in the earlier chapters, but now we may use it for an analysis of another series of biological phenomena. Its sac-like body consists of two cell-layers; the outer one is concerned primarily with offense and defense, while the inner layer is made up of digesting or nutritive elements.
Word Of The Day