This investigation was performed in order to analyze the basic relationships between the archenteron roof and the overlying ectoderm in primary induction in the Cynopus (Triturus) pyrrhogaster embryo.The part of the archenteron roof that is active in inducing capacity extends linearly after invagination at the speed of 0.15 mm per hr at 23°C until stage 13b. The period of contact at each position of the presumptive neuro-ectoderm with the active archenteron roof could be estimated by the formula described in the Discussion.Pieces of the presumptive neuro-ectoderm were isolated from gastrulae at three developmental stages and cultured separately in Holtfreter solution after being divided caudo-cranially into 4 parts. The result showed that some of them were able to differentiate into neural tissues even in the mid-gastrula stage and that the presumptive neuro-ectoderm acquired the capacity to differentiate into neural tissue along a caudocranial axis from the part adjacent to the blastopore during gastrulation.It could be estimated that 3 hr of contact with the active archenteron roof is sufficient for the presumptive neuro-ectoderm to differentiate into neural tissue.The present study also showed that the neuralizing capacity of the whole prospective neuro-ectodermal area has already been determined before the end of stage 13, i.e., within less than 14 hr after first contact of the ectoderm with the active archenteron roof at 23°C.Since SPEMANN'S pioneering work on the organizer, many attempts have been made t o analyze the relationships between the actor (organizer) and the reactor (ectoderm) in the primary induction of the amphibian embryo. For neural differentiation in primary induction, the temporal relationship between the invagination of the archenteron roof and the response of the overlying ectoderm is important (EYAL-GILADY, 1954), and a certain period of interaction between the competent ectoderm and the inductor is indispensable (JOHNEN, 1956: TOIVONEN, 1958: SUZUKI, 1968b: NODA et al., 1972.
The Japanese giant salamander (Andrias japonicus) is a near threatened species endemic to western Japan and is strictly protected by law. However, available information regarding the genetic diversity and genetic structure in this species, essential for its effective conservation, has been limited. We developed four microsatellite markers from A. japonicus and characterized these markers for two populations of this species, as well as for some captive Chinese giant salamanders (A. davidianus) of unknown original locality or localities. These markers, showing expected heterozygosities of 0.00-0.50 in the former and 0.63-0.89 in the latter, will be useful in documenting population genetic properties for each of the two species.
Mitotic activity and cell proliferation of newt (Triturus pyrrhogaster) embryo were examined with special reference to primary induction.Mitotic activity o f gastrula ectoderm gradually decreases during gastrulation.The ectoderm, which is isolated from mid-gastrula (stage 12b) and cultured in vitro, also shows gradual decrease in mitotic activity during cultivation and the mitotic activity steeply decreases after 48 hr. The ectoderm cultured with heterologous inductor (GPL-extract) shows a temporal suppression in mitotic activity. The ectoderm of the whole gastrula also shows a regional suppression where it is in contact with the chorda-mesoderm.The number of the ectodermal cells increases about 2 times after 24 hr culture and to more than 3 times after 48 hr culture. Accordingly it is certain that the majority of the ectodermal cells divides at least one time in the course of 48 hr.Histological examination of the ectoderm cultured together with the inductor reveals that differentiation o f undifferentiated ectoderm to neural tissues is accomplished at least within 48 hr after cultivation with the inductor.The present examination shows the possibility that the mitotic activity of the ectoderm may be temporarily suppressed by the inductor and that it then decreases along with neural cell differentiation after recovery of the activity.The results also suggest that the determination of undifferentiated ectoderm to neural tissues occurs before the second cell division after the contact with the inductor and the events occurring during the first cell cycle after activating by the inducing stimulus are critical for the primary induction.
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