102 



EMBRYOLOGY 



BARRIER 



NEURULA 



Fig. 50. Two hearts in a single embryo. The left figure shows the two 

 blocks of heart mesoderm which normally fuse to form a single heart. If a 

 barrier is placed between the two halves of the heart mesoderm, they fail to 

 fuse and each half develops into a heart, as in the right figure. 



44). There is a limb field in the mesoderm; a lens field in the ectoderm; an 

 ear field, also in the ectoderm; a field for the olfactory placode, again in the 

 ectoderm; a heart field in the mesoderm; a hind-limb field in the mesoderm. 

 All of these fields that have been examined show a labile type of organiza- 

 tion which can be modified by experimental treatment. 



The nature of a field is very difficult to understand, but it does have 

 certain quantitative characteristics which have been demonstrated particularly 

 well in the case of the ear field. The ear arises as an ectodermal vesicle called 

 the otocyst. The potencies of the ectoderm around the ear region may be 

 explored by transplanting small pieces of the region to the ear region of 

 another embryo. Usually these pieces of ectoderm will form a typical pear- 

 shaped otocyst. Careful exploration of the whole region around the normal 

 site of the ear indicates that a greater number of normal ear vesicles is obtained 

 if transplants are made from or near the normal ear region. On the other 

 hand, if transplants are made from areas farther and farther away from the 

 normal ear region, fewer vesicles are obtained and these vesicles are more 

 and more abnormal in structure. The field thus appears to vary in potency 

 in a quantitative way, so that in the center of the field perfect ear vesicles 

 are obtained in about 100 percent of the cases, whereas farther out from this 



