94 



EMBRYOLOGY 



NEURAL TUBE 

 SOMITE 

 NOTOCHORD 



PRONEPHRIC 

 DUCT 



NASAL PIT 



PRONEPHRIC 

 DUCT 



SALT SOLUTION 



Fig. 45. Self-differentiation of structures in 

 foreign environments. Above: An otocyst which 

 differentiated from the transplanted presumptive 

 otocyst of a neurula. Upper right: A nasal pla- 

 code which differentiated from the transplanted 

 presumptive nasal placode of a neurula. Right: 

 A functional heart derived from mesoderm 

 taken from the heart region of a neurula. 



posterior in the ectoderm will form the otic vesicle and will continue to 

 differentiate in the flank region. From the ventral region of the neurula a 

 fragment consisting of mesoderm together with a little ectoderm will, upon 

 removal to a salt solution, round up to form a sphere. Inside this sphere a 

 perfect heart will develop, and this heart will be functional in the sense that 

 rhythmic contractions will take place (Fig. 45). One could go on and map 

 out the whole neurula in this way and thus demonstrate that each major struc- 

 ture — it should be emphasized that these are major structures, such as the 

 heart, limb, ear — is determined at this stage of development. Figure AG illus- 

 trates how complete is the differentiation of the eye after the presumptive 

 eye has been transplanted to the body of another embryo. This property of 

 an organ to differentiate according to its presumptive value when transplanted 

 is called self-differentiation. After a presumptive structure has been deter- 

 mined by the process of embryonic induction, it will self-differentiate even 

 though it is transplanted to a foreign environment, where it may be exposed 

 to other inductors. 



