352 CELL HEREDITY 



Then one or several of these blastulae were chosen, and groups of nuclei 

 from each were transferred to oocytes, and the process was repeated. 

 At each "subculture, ' most of the material was allowed to develop as far 

 as it would go in order to score the phenotypes, and only a few blastulae 

 were sacrificed for further cloning. Similar studies have been carried out 

 with the related genus, Xenopus, with similar findings. 



There is much greater homogeneity of developmental stage reached in the 

 subclones than in the original set of nuclei taken from the late stages of de- 

 velopment. For example, when nuclei are taken from various regions of late 

 gSLStrulae oi Rana pipiens (such as chorda mesoderm, presumptive medul- 

 lary plate, or endoderm ), most of them give rise to embryos which are block- 

 ed at early stages even before gastrulation; but quite a range of response 

 levels have been observed. The group of embryos which develop by clonal 

 subculture from one of these nuclei, on the contrary, are fairly uniform 

 in distribution of stages to which they develop. Similarly, the group 

 of embryos developing trom a different one of these original transferred 

 nuclei is quite uniform, but there may be drastic differences in level of 

 development between the two sets (Figure 11.21). 



These results demonstrate the existence of great heritable stability on 

 the part of the alterations experienced by the original nuclei taken from 

 late embryonic stages. These alterations, which differ from nucleus to 

 nucleus, are transmitted regularly in subsequent nuclear divisions. 



It is evident that little information is as yet available on the origin of 

 the initial alterations in nuclei from late embryonic stages. Nonetheless, 

 the stability of these alterations under subsequent cloning conditions is a 

 very important finding in its own right. 



From the analytical viewpoint, this kind of experimentation has serious 

 limitations, involving as it does a highly complex set of interacting 

 variables. The transferred nucleus brings with it not only the chromo- 

 some complement but also the nucleolus, nuclear membrane, and at- 

 tached bits of cytoplasm. Although the bulk of cytoplasm transferred is 

 small, the possibility exists that it does not represent a random sample, 

 but rather a region of specialized function attached to or adjacent to the 

 nucleus. The results cannot be fully accepted as evidence of chromo- 

 somal effects, nor even of nuclear effects, without correlated evidence 

 from an independent line of experimentation. 



CONCLUDING REMARKS 



In this chapter we have examined a few of the current lines of research 

 which have been particularly fruitful in providing new information, new 



