. Analysis of development. Embryology; Embryology. The Role of Nucleus and Cytoplasm o O. Fig. 26. Diagram to illustrate the independence of final body size from initial mass of cytoplasm of egg. A half-egg of a salamander produces an em- bryo of half normal size; during later stages the lai'va grows to approximately the same size as a larva from a whole egg (^). A large egg produced by a polyploid female (c) gives rise to a large em- bryo; slower growth during later feeding stages re- duces final size to normal. phosis, at the age of about three months (Fig. 26; Spemann and Falkenberg, '19, F
. Analysis of development. Embryology; Embryology. The Role of Nucleus and Cytoplasm o O. Fig. 26. Diagram to illustrate the independence of final body size from initial mass of cytoplasm of egg. A half-egg of a salamander produces an em- bryo of half normal size; during later stages the lai'va grows to approximately the same size as a larva from a whole egg (^). A large egg produced by a polyploid female (c) gives rise to a large em- bryo; slower growth during later feeding stages re- duces final size to normal. phosis, at the age of about three months (Fig. 26; Spemann and Falkenberg, '19, Fig. 1, PL 1; Fankhauser, '45a). Similarly, tadpoles developing from large and small eggs spawned by the same female frog (Rana pipiens), with a volume ratio of to 1, reached approximately equal sizes after 53 days of larval growth (Briggs, '49). Fur- thermore, larvae raised from the large eggs produced by older triploid and tetraploid axolotls, regardless of the number of chro- mosomes which they contain, are at first con- spicuously larger than the controls but will grow more slowly later on (Fankhauser and Humphrey, '50). Final body size is clearly independent of the initial mass of cyto- plasm of the egg. NUCLEUS Development without Nucleus. Various methods have been applied to induce develop- ment of eggs in the complete absence of fimctional nuclear material (Fig. 27, Table 5). Essentially similar results have been ob- tained with eggs of both sea urchins and amphibians. Cleavage may proceed more or less regtilarly, usually in the presence of cytasters or of sperm asters not associated with functional sperm nuclei, but sometimes in the complete absence of an achromatic mitotic apparatus. Development comes to an end at various stages but may proceed to the formation of a blastula without blasto- 129 coele (sea urchin), a partial blastula with unsegmented areas in the vegetal region (frog), or an apparently normal blastula (axolotl). Although such blastulae may sur- viv
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Keywords: ., bookcentury1900, bookpublisherphiladelphi, booksubjectembryology
