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 1.73 

 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- 

 vive for days, in no instance has even an 

 attempt at gastrulation been observed. This 

 is true also of blastulae of newts that con- 

 tain both nucleated and non-nucleated areas. 

 There is no doubt that some fundamental 

 process or processes beginning at gastrula- 

 tion cannot take place except in the presence 

 of nuclei in all, or at least a majority of, 

 the cells. 



Fig. 27. Development of non-nucleated eggs and 

 egg fragments (see Table 5). 



a, Unfertilized egg of sea urchin (Arbacia) is 

 fragmented by centrifuging. Lower non-nucleated 

 half stimulated to parthenogenetic development. 

 Cleavage with cystasters to blastula without blasto- 

 coel (after Harvey, '36). 



b, Removal of egg nucleus from fertilized egg of 

 axolotl by puncture to produce haploid development 

 (androgenesis). In a single case, the sperm nucleus 

 degenerated, but the sperm aster functioned in 

 cleavage producing a normal, advanced blastula 

 (after Stauffer, '45). 



c, Fertilization of egg of Rana pipiens with sperm 

 of R. catesbiana, previously exposed to heavy dose 

 of x-rays, followed by removal of egg nucleus. The 

 sperm nucleus remains inert, the sperm aster fimc- 

 tions in cleavage leading to formation of partial 

 blastula. Remnants of degenerated sperm chromatin 

 visible in some cells (after Briggs, Green and King, 

 '51). 



