INDEPENDENT EFFECTOES 59 



heart of Ascidia is relatively slow, 1.76 to 2.12 centimeters 

 per second, a rate more in accord with muscle transmission 

 than with nerve transmission (Hecht, 1918). The heart 

 of the tunicate, therefore, appears to be like that of the 

 adult vertebrate, an independent effector complicated in 

 a secondary way by the presence of nerve cells. 



According to the investigations of Lewis and Lewis 

 (1917) the amnion of the embryo chick is an organ that 

 exhibits all the characteristics of an independent effector. 

 This membrane is composed of a single layer of mesen- 

 chyme cells overlaid by a single layer of epithelial cells. 

 The mesenchyme cells differentiate into smooth muscle 

 cells that undergo definite contraction as early as the 

 fourth or fifth day of incubation. So far as is known, 

 there is no nerve supply to the amnion. Hence contrac- 

 tions in the smooth muscle layer in this organ must be 

 excited by the direct stimulation of its fibers. This view 

 is supported by the fact that in tissue cultures made from 

 the amnion there can be easily found in the outgrowing 

 cells after a day or two groups of smooth muscle fibers or 

 even single fibers that exhibit rhythmic contractions. 

 These are absolutely devoid of nervous connections and 

 must, therefore, be stimulated directly, a conclusion that 

 is supported by the fact that after they have ceased to 

 contract rhythmically they can be re-excited to this activ- 

 ity by washing them with a drop of fresh culture medium 

 to which an excess of calcium has been added. There is, 

 therefore, every reason to believe that the smooth muscle 

 layer of the amnion of the chick is an independent effector. 



The instances of this peculiar type of tissue thus far 

 noticed in animals above the sponges have been taken 

 from the higher animals and almost entirely from the 

 vertebrates. This is due to the fact that since these 



