In the lower plants, such as the mosses, the liverworts, the moulds, and the unicellular forms, regeneration also takes place. Vöchting has shown that pieces from any part of the thallus of a liverwort[8] produce new plants. If a cross-piece is cut off, there appears a small outgrowth from the middle of the anterior cut-edge, as shown in Fig. 9, _A_, _A²_, that gradually enlarges to form a new thallus. It will be seen from the figures that the whole anterior edge does not grow forward, but a new thallus arises from a group of cells at, or near, the anterior edge. These cells are the least-differentiated cells in the piece, and have softer cell walls than have the other cells. [Illustration: FIG. 10.--After Pringsheim. _A._ A piece of seta of sporophore of _Hypnum cupressiforme_, sending out protonema-threads. _B._ Longitudinal section of a piece of the seta of sporophore of _Bryum cæspitosum_. _C._ Piece of same of _Hypnum cupressiforme_. Moss-plant arising from new protonema. _D._ Piece of same of _Hypnum serpens_ with protonema and moss-plant arising from it.] Pringsheim has shown that if a piece of the stalk of the sporangium of certain mosses is cut off, it produces at its ends thread-like outgrowths which are like the protonema-stage of the moss, and from this protonema new moss-plants may arise (Fig. 10, _A_, _B_, _C_, _D_). Braefeld has obtained a somewhat similar result in one of the moulds, in which a piece of the sporangium stalk gives rise to a mycelium from which new sporangia may be produced. _REGENERATION IN EMBRYOS AND EGGS_ Regeneration takes place not only in adult organisms, but also in embryos, and larvæ of many animals. It is often stated that the power of regeneration is more highly developed in embryos than in adults, but the facts that can be advanced in support of this view are not numerous. One of the few cases of this sort known to us is that of the leg of the frog, that does not regenerate, while the leg of the tadpole is capable of regenerating. [Illustration: FIG. 11.--_A._ Blastula of Sea-urchin. Dotted lines indicate where pieces of wall were cut off. To the right are shown stages in the development of these pieces. _B._ Two-cell stage of egg of sea-urchin. One blastomere isolated. Its development shown in figures to right of _B_. _C._ Fertilized but unsegmented egg. Dotted line indicates where it was cut in two. Upper row of figures to right shows development of nucleated piece; lower row shows the fertilization and development of non-nucleated piece.]