' REPRODUCTION 135 



the injection of adenosine into a chick egg, but to date no enzyme has 

 been induced which is not normally present to some extent in the em- 

 bryo. Adult tissues show marked differences in their enzymatic activities, 

 differences which might be the result of "adaptations" comparable to 

 those seen in bacteria. Adaptive changes in enzymes, however, are tem- 

 porary and reversible, whereas differentiation is a permanent, irreversible 

 process. Cells may lose some of their morphologic characteristics but they 

 retain all of their biochemical specificities. 



Some interesting data bearing on the problem of morphogenesis have 

 been obtained recently by Briggs and King of the Lankenau Institute. 

 They have been able to transplant a nucleus from one of the cells of an 

 early blastula of a frog into an enucleated egg. This egg will subse- 

 quently cleave, gastrulate, and develop normally. However, if a nucleus 

 is taken from a cell of the late gastrula— from a chorda-mesoderm or 

 midgut cell— and transplanted into an enucleated egg, abnormal develop- 

 ment results. Development is arrested in the blastula or gastrula stage. 

 Transplanted chorda-mesoderm nuclei result in embryos with deficient 

 or absent nervous systems and transplanted midgut nuclei form embryos 

 with thin or absent epidermis and no nervous system (Fig. 6.12). These 

 experiments indicate some change in the intrinsic differentiative proper- 

 ties of the nuclei as cleavage and development proceed. Nuclei taken 

 from even later stages in development cannot function in cleavage; an 

 enucleated egg receiving such a nucleus does not develop at all. The 

 nature of this nuclear specialization is unknown, but the loss of differ- 

 entiative potentialities bears some relationship to the site of the embryo 

 from which the nucleus was derived. 



Evidence of a different type of differentiation mechanism has been 

 obtained from experiments in which microsurgical instruments are used 

 to cut out a bit of tissue from one embryo and transplant it to another. 

 For example, when a piece of the dorsal lip of the blastopore of a frog 

 gastrula is implanted beneath the ectoderm of a second gastrula, the 

 tissue heals in place and causes the development of a second brain, 

 spinal cord and other parts at the site, so that a double embryo or closely 

 joined Siamese twins results (Fig. 6.13). Many tissues show similar abili- 

 ties to organize the development of an adjoining structure. The eyecup 

 will initiate the formation of a lens from overlying ectoderm even if it is 

 transplanted to the belly region, where the cells would normally form 

 belly epidermis. Such experiments indicate that development is a co- 

 ordinated series of chemical stimuli and responses, each step regularly 

 determining the succeeding one. The term "organizer" is applied to the 

 region of the embryo with this property and also to the chemical sub- 

 stance given off by that region which passes to the adjoining tissue and 

 directs its development. There is evidence which suggests that organizers 

 are nucleoproteins. 



It had been widely accepted that organizers can transmit their 

 inductive stimuli only when in direct physical contact with the reactive 

 cells. However, evidence from experiments by Victor Twitty of Stanford 

 indicates that induction can occur by diffusible substances which are 

 capable of affecting the induction of a second tissue at a distance. Twitty 



