136 



GENERAL CONCEPTS 



Nuclcu-S removed a.nd- 

 implantcd via. micropipebtc 



Blastulsi- 



Enuclealed egg 



Nucleus of 

 chorda_me.sod.crm 

 cell impla.nte.d 



La.te oa-st-rula. 



En.uclea.ted e^^ 



Normal embryo 



Development s:opS 

 in blaslixla. cr 

 ga-strula. sta-ge; 

 abnoT-mal embryo 

 U/ith. def iciei^t 

 nervouLS syslem. 



Deuelop'm.cn.t stops 

 in blastula. ot 

 ga-st-rula. stage-, 

 abnoTr^rrLaJ- cmbrj^O 

 with. deficie.n.t 

 <^pidermis and 

 nervous system. 



Figure 6.12. Diagram of experiments with the transplantation of nuclei to enu- 

 cleated eggs. See text for discussion. 



grew small groups oi frog ectoderm, mesoderm and endoderm cells in 

 tissue culture and found that ectoderm alone would never differentiate 

 into nerve tissue. Ectoderm cells placed in a medium in which mesoderm 

 cells had been grown for the previous week, did differentiate into 

 chroma tophores and nerve fibers. No comparable differentiation occurred 

 when the ectoderm cells were placed in comparable cultures of endoderm 

 cells. Twitty concluded that inductor tissues, such as chorda-mesoderm, 

 contain and release diffusible substances which are capable of operating 

 at a distance and inducing the differentiation of ectoderm. This sub- 

 stance has been tentatively identified as nucleoprotein. 



Evidence that steroids, as well as nucleoproteins, may play a morpho- 

 genetic role in development has been obtained by Dorothy Price, of the 



