92 EMBRYOLOGY 



Fig. 43. A neurula at a stage where most 

 of the major structures will self-differentiate 

 from their fields when these are transplanted 

 to a second neurula. The fields used in the 

 experiments described in the text are those 

 of the nasal placode, otocyst, heart, and 

 limb. As can be seen, there are no visible 

 signs of the various fields. Their presence 

 can be demonstrated only by cutting out 

 regions with known presumptive values. 



mination in the neurula stage may be demonstrated by some experiments on 

 the amphibian neurula. 



When a piece of tissue is tested for determination, it is usually trans- 

 planted to the flank region of another embryo. In the flank region the tissue 

 is exposed only to the action of the yolk, which is rather inert. Thus the trans- 

 planted tissue develops independently of other structures and there is little 

 possibility of induction by the nervous system or the roof of the archenteron. 

 The purpose of such a method is to see what the tissue in question can do 

 when all outside influences are removed. Any differentiation obtained by this 

 technique is caused by factors within the transplanted tissue itself. For this 

 reason the test is called a test for self -differentiation and the result tells us 

 whether or not a given tissue is determined. Thus, in the neurula stage a disk 

 of tissue containing both the ectoderm and the mesoderm may be removed 

 from about the middle of the embryo, and if this tissue is transplanted to the 

 flank region of another embryo, it will develop into a perfect forelimb (Fig. 

 44). Further, if nerve connections are established, the forelimb will function 

 perfectly. 



The differentiation of this disk of ectoderm and mesoderm is perfect, and 

 the muscles, bones, cartilage, and skin of the forelimb differentiate in spite 

 of the fact that there are no visible differences between the cells of this limb 

 region and some adjacent region. This disk of tissue is unquestionably deter- 

 mined to form a forelimb no matter where it is placed. It can be transplanted 

 to the top of the head, to the back, or anywhere else and it will always form 

 a forelimb. 



In the same way a certain piece of ectoderm may be removed from the 

 head region of the neurula and tested in the same way as the limb region. It 

 develops into a perfect nasal placode (Fig. 45). Another region a little more 



