Dorsal (D), lateral (L and R), and ventral (V) portions of the endoderm of blastulae ofAmbystoma mexicanum of different age (stages 8 to 10) were combined with ectodermal caps of stage 8 blastulae. All V and most L and R portions induced only ventrocaudal mesodermal structures - "ventral" type of mesoderm induction. Almost all D portions induced much more voluminous structures of predominantly axial character - "dorsal" type of mesoderm induction. The difference in mesoderm-inducing capacity of the dorsal as against the lateral and ventral endoderm is probably purely quantitative in character. The dorsal endoderm exhibits a pronounced dominance in mesoderm-inducing capacity. During the early symmetrization of the amphibian egg it is apparently especially the presumptive dorsal endoderm that becomes endowed with strong mesoderm-inducing properties.A comparison of the results obtained with endodermal portions of blastulae of different age showed that the mesoderm-inducing capacity first begins to decline in the dorsal endoderm (around stage 9), subsequently in the lateral, and finally in the ventral endoderm (at stage 10). At stage 10 the dorsal endoderm no longer has mesoderm-inducing capacities.In the recombinates there is a striking correspondence between the regional differentiation of the mesoderm and that of the endoderm. The latter differs markedly from the presumptive significance of the various endodermal regions in the normal embryo.Primordial germ cells, which constitute a characteristic component of the "ventral" type of mesoderm induction, can be induced not only by ventral, but also by lateral and to some extent even by dorsal endoderm. The development of primordial germ cells from the ectodermal component of the various recombinates indicates that in the urodeles the origin of the primordial germ cells differs markedly from that in the anurans.

The animal and the vegetative side of 15 embryos ofXenopus laevis were studied from the 5th cleavage to gastrulation by means of time-lapse cinematography. The duration of cleavage cycles, defined for the embryo as a whole as the period between the earliest blastomere divisions of one cycle to those of the next, varies quite a lot between individual embryos, both with respect to synchronous and lengthened cycles. Cycle lengthening may start at either cycle 10, 11 or 12. Cycle 13 deviates from the individual rhythm, and moreover its duration is inversely correlated with the period elapsing from the beginning of this cycle to the onset of gastrulation which occurs in cycles 14 or 15. In each cleavage cycle, the regional sequence of first blastomere divisions is visible on films as a "cleavage wave" runming over the animal cap. The direction of the waves varies in different embryos during the synchronous period but begins to change from cycle 10 onwards, resulting in a similar direction in most embryos prior to gastrulation: from the ventral/left to the dorsal/right half. This change reflects an asymmetry in the lengthening of the cycles in the animal cap: more dorsally than ventrally, and more on the right than on the left. The possible significance of the results for the timing of gastrulation and for the pattern of the future embryo is discussed.

The animal and the dorsal side of five embryos of Xenopus laevis were studied in detail from the 7th to the 13th cleavage by means of time-lapse cinematography. At each cleavage the regionally ordered sequence of blastomere divisions is visible in the films as a "cleavage wave", propagating about three times slower in the dorsal than in the animal view. In the dorsal view the waves run in an animal-vegetal direction, initially with a left-to-right deviation and in later cleavages converging on the region of the future blastopore. The lengthening of cleavage cycles begins at cycle 8 on the dorsal side, just above the future blastopore. From cycle 9 to 11 nearly equal lengthening occurs in each cycle at all animal-vegetal levels. In general, cycles lengthen a little more in median than in lateral sectors and a little more in right than in left sectors. Cycle 12 is longest in the sector above the future blastopore and shortest in the animal region. The results show that the initial pattern of a regionally ordered sequence of cleavage cycles of equal duration changes into a pattern of cycles of different durations as a result of gradual cycle lengthening, starting in the region just above the future blastopore and spreading in animal direction. The results are compared with data on the cleavage cycles of isolated blastomeres, and the possible relation with the induction of the mesoendoderm occurring during the stages studied is discussed.

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In early neurulae of Triturus alpestris the cranio-caudal distribution of differentiation tendencies in the presumptive prosencephalon was tested. Small grafts made from a median strip excised from the region of the neural plate and transverse neural fold were transplanted to a pocket made under the cranio-ventral ectoderm of early neurulae. Grafts made from the middle portion of the strip showed almost exclusively eye differentiation, whereas more cranial as well as more caudal grafts showed relatively more telencephalic differentiation, and only little diencephalic differentiation. The results indicate that the process of pattern formation in the central region of the prospective prosencephalon is advanced with respect to the surrounding areas.