By Professor Dr. Ernst Hadorn (auth.)
Although this can be essentially a translation of the second one German variation released in 1970, newer experimental findings have, in different situations, been integrated into the textual content. moreover, we've attempted to give an explanation for many of the experiments, and their attainable interpretations, in a extra particular manner. i'm very thankful to Dr. David Turner; as well as translating the textual content, he was once capable, due to his adventure in developmental biology, to signify a few advancements during our collaborative discussions. Zurich, Spring 1974 ERNST HADORN Preface to the second one German version The guideline of the 1st version continues to be in strength. that's, the equipment and result of developmental examine are intro duced at any place attainable by using experiments on am phibians. in spite of the fact that, the scope of the fabric has been considerably ex panded in newly brought chapters at the migrations and affinities of somatic and germ cells in addition to at the motion of genetic components in early improvement. those are fields of analysis that are on the heart of ultra-modern learn. additionally, a number of new findings were included into the textual content. the writer hopes that this little booklet will proceed to facilitate figuring out of fascinating study difficulties, for the layman in addition to for the instructor and pupil of biology.
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8b). In the newt there are twelve such pairs. Each chromosome within a pair is in turn subdivided into two sister chromosomes, making a group of four chromosome strands in all, called a tetrad. The chromosomes of the tetrad are then distributed into future egg and sperm cells by means of two maturation divisions; this is accomplished in such a way that each mature gamete receives only a single (haploid) chromosome set of nonhomologous chromosomes. The mechanism of the meiotic divisions, which will not be described in detail here, assures that each individual chromosome, and hence each gene, is represented only once in the new set achieved by random assortment of paternal and maternal chromosomes.
8a). In later stages the ribosomal genes no longer detach from the nucleolus organizer but remain attached at the constriction shown in Figs. 9f and 9g and function at that spot. In Xenopus there are approximately 600 identical functional units arrayed in tandem along the DNA strand, all of them coding for ribosomal RNA. The second gene which has been important in elucidating the function of the lampbrush stage was discovered in the axolotl (Ambystoma mexicanum). In a strain of these laboratory animals larvae were observed which show somewhat slower growth than normal animals.
8a). There are several hundred of these oocyte nucleoli in the newt. In the clawed frog, the oocyte nucleoli come to number more than a thousand while the other cell types usually have only two nucleoli per cell. Thus provided with a rich ribosomal dowry, the egg cell is ready, once the lampbrush chromosomes have regressed, to enter the phase of fertilization and early development. With this general picture of the genetic activity of lampbrush chromosomes in mind, we now want to introduce two specific examples to show that gene activity occurring during the lampbrush stage is vital for the first steps in the life of the next generation.