The Role of Nucleus and Cytoplasm 



143 



J. A. Moore ('47b, '48) transplanted pieces 

 of ectoderm from nonviable gastrulae of R. 

 pipiens 2 X i?. sylvati'^,7 $ to normal neu- 

 rulae of R. palustris or Arnblystoma macu- 

 latum and found that formation of neural 

 tissue and sense organs may be induced in 

 the graft; however, the competence of the 

 hybrid ectoderm to form these structures is 

 definitely reduced. On the other hand, grafts 

 of pieces of dorsal lip from hybrid gastrulae 

 to normal ones induce poorly differentiated 

 secondary embryos in a limited number of 

 cases. Moore proposed the interesting hypo- 

 thesis that the foreign genes of R. sylvatica 

 lower both the competence and the inductive 

 power of the hybrid tissue by competing with 

 the R. pipiens genes for a svibstrate present 

 in limited amount and forming an "analog" 

 that cannot take the place of the substance 

 formed by the normal pipiens cells. 



Studies of the metabolism of arrested hy- 

 brid gastrulae (Barth and Jaeger, '47; Gregg, 

 '48) have shown a marked redviction in res- 

 piration and glycolysis which indicates that 

 one of the effects of the R. sylvatica chromo- 

 somes in a R. pipiens egg is the partial block- 

 age of at least one step in the glycolytic 

 chain. In the R. esculenta 2 X i?. temporaria 

 $ hybrid, synthesis of cytoplasmic RNA is 

 sharply decreased, while an excess of RNA 

 seems to accumulate in the nucleus (cf. 

 Brachet, '52). 



Development of viable reciprocal hybrids. 

 As a rule, the rate of cleavage and gastrvila- 

 tion of hybrid eggs is maternal. In the re- 

 ciprocal crosses R. pipiens X R. palustris, an 

 effect of the sperm has been detected as early 

 as neurulation, producing an intermediate 

 rate of development from this stage on (J. 

 A. Moore, '41). The hybrids between T. 

 taeniatus (or T. palmatus) and T. cristatus 

 are completely maternal in appearance as far 

 as the limb bud stage, with the possible ex- 

 ception of the size of the optic vesicle. The 

 first effect of paternal genes is visible in the 

 total length of the embryo and in the pig- 

 mentation of the tail fin (Hamburger, '36). 

 Hybrids between the three species of Cali- 

 fornia newts show an intermediate condition 

 in the development of the pigmentation, bal- 

 ancers and dorsal fin (Twitty, '36). 



Malformations of the hindlimbs are typical 

 for all hybrids between T. taeniatus or T. 

 palmatus 2 and T. cristatus $ but are ab- 

 sent in the reciprocal combinations, possibly 

 because of a difference in the egg cytoplasm 

 of the two species (Hamburger, '35). In the 

 same combination, as well as in hybrids be- 



tween T. taeniatus and T. alpestris, Pariser 

 ('32, '36) noted an abnormal sex ratio (over 

 98 per cent females) and a subnormal de- 

 velopment of the ovaries, with a deficiency 

 of growing oocytes. These investigations 

 should be continued since they offer a new 

 line of attack on the complex problem of sex 

 determination in amphibians. 



Inviability of hybrids between different 

 geographical forms of Rana pipiens. While 

 R. pipiens from most localities produce viable 

 hybrids with R. palustris, crosses between 

 northern and southern forms within the spe- 

 cies give rise to abnormal and frequently 

 nonviable embryos. Eggs of a northern race 

 (e.g., Vermont) fertilized with sperm from a 

 southern race (e.g., Florida) show retarda- 

 tion of development, extreme head enlarge- 

 ment and defects in the circulatory system. 

 The reciprocal combination produces reduc- 

 tion in head size, frequent fusion of olfactory 

 pits and eyes and absence of mouth. The 

 cross Vermont 2 X Mexico $ results in 

 frequent exogastrulation and beginning of 

 cytolysis in the gastrula and neurula (J. A. 

 Moore, '47a). The existence of differences in 

 the egg cytoplasm, which is suggested by 

 these opposite effects produced in reciprocal 

 diploid hybrids, was also demonstrated by 

 Porter ('41, '46), who removed the egg nu- 

 cleus from cross-fertilized eggs. The haploid 

 androgenetic "hybrids" showed a more ex- 

 treme effect of the cytoplasmic factors but 

 also an early activity of the "foreign" sperm 

 nucleus. 



HAPLOID, ANDROGENETIC HYBRIDS 



The crucial experiment, combination of a 

 sperm nucleus of one species with the egg 

 cytoplasm of another in the absence of the 

 egg nucleus, was first carried out by Boveri 

 (1896, '18) by cross-fertilizing fragments of 

 sea urchin eggs. Table 8 shows that of the 

 several combinations obtained by him and 

 other investigators only two gave rise to 

 plutei. Paracentrotus cytoplasm with Psam- 

 mechinus nucleus produced a larval skeleton 

 with typically paternal characteristics; and 

 the reciprocal combination showed certain 

 maternal characteristics which may have 

 been due to the effect of the Psammechinus 

 cytoplasm or, possibly, to the influence of 

 unfavorable environmental conditions (Hoer- 

 stadius, '36). The importance of the nucleus 

 was also demonstrated by combining the 

 skeleton-forming micromeres of an andro- 

 genetic hybrid with the presimiptive ecto- 



