FIELDS AND THEIR PROPERTIES 



101 



Fig. 49. Comparison of the limb 

 disk with the sea urchin egg. The 

 sea urchin egg is the germ for the 

 pluteus larva; the limb disk is the 

 germ for the limb. Both have a 

 polarity as regards the structure 

 which they form. In both cases the 

 polarity may be altered by experi- 

 mental procedures. A = animal 

 pole; V = vegetal pole. In the limb 

 disk, d = dorsal ; v = ventral ; p = 

 posterior, and a = anterior. 



LIMB DISK 

 SEA URCHIN EGG 



PLUTEUS LARVA 



may be rearranged and they will still form a perfect limb. Because the limb 

 region is so labile it is called a limb field at this stage of its differentiation. 

 One of the characteristics of an organ field is the property of forming two 

 or more entire organs from a single field. 



Fields and their properties 



The limb field is not an isolated case of an organ field. Other structures 

 in the neurula behave in the same way — the heart, for example. In the 

 neurula stage the mesoderm that will give rise to the heart is ventrally located 

 and is in two parts (Fig. 50). The single heart in vertebrates regularly forms 

 from the fusion of two masses of mesoderm. Although these two masses of 

 mesoderm normally fuse to form a single tubular heart, each one may by 

 itself form a perfect, functional heart. The blood from the veins then goes 

 through two separate hearts instead of a single one. This result may be 

 achieved merely by inserting a barrier between the two masses of heart 

 mesoderm so they cannot fuse. Each one of these mesodermal regions con- 

 stitutes a heart field. If each one of these fields is cut into halves, two hearts 

 are obtained from each half. As many as four hearts may develop from the 

 mesoderm which would normally form a single embryonic heart. 



The neurula, then, is composed of a number of these organ fields (Fig. 



