Growth characteristics of the total brain and of several individual brain regions are described in Apteronotus leptorhynchus. The increase of brain volume relative to total length growth results in a sigmoid growth curve, in which three phases – proportional growth, positive-allometric and negative-allometric growth – could be distinguished. This type of enlargement of the total brain is due to the differential volume increase of individual brain regions and to the increase in the volume of the rhombencephalic structures in particular. Pros- and mesencephalic regions mainly grow in accordance with the overall enlargement of the total brain throughout development, whereas rhombencephalic and certain nuclear structures show such isometric growth only with the onset of the juvenile period. The results are interpreted in the view of functionality of the individual brain regions.
Embryos of the urodele Ambystoma mexicanum were used in time set experiments for the analysis of ectoderm-chordamesoderm interactions in "primary embryonic induction". The influence of chordamesoderm-age (CM-age), ectoderm-age (E-age), ectoderm-mass (E-mass) on differentiation processes were investigated. The results revealed that early interactions exist, which, however, seem to be dominated by the ectoderm in various respect: a) the segregation of the CNS is due to the autonomous change in ectoderm competence phases; b) based on the temporal sequence of competence phases, influences are exerted by the ecto-neuroderm which support the chordamesoderm in its self-differentiation capability to chorda and other mesodermal tissues. Consequently, reciprocal impulses from these newly differentiated tissues are warranted (feedback to ectoderm); c) coordination of the cranialization effect and corresponding ectodermal competence phase is achieved; d) the inductive efficiency of the chordamesoderm is strongly supported.Three statistical tests (regression, correlation, information analyses) were conducted (dependant variables: quality and quantity of the differentiated structures; independant variables: experimental factors). All three tests equally rendered statistical significance for the following order of decreasing importance the experimental factors play in induction and differentiation processes: E-age, E-mass, CM-age.
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