ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 73 



double monster, apparently by the production of two D-cells in 

 place of one (see p. io8). In such cases the direction of spiral 

 cleavage is reversed in the right-hand D-cell and all other cells on 

 the right side of the plane of bilateral symmetry. 



In the Echinoderms, most of the larval forms are asymmetrical, 

 in that the left, but not the right, coelomic pouch acquires a water- 

 pore placing it in communication with the exterior. Further, the 

 fates of the various right and left coelomic pouches are very differ- 

 ent. As a result, the hydrocoel and the rudiment of the body of 

 the adult Echinoderm are formed on the left side of the body of 

 the larva. It will be best to postpone the analysis of conditions 

 in this group until the state of affairs in Vertebrates has been 

 considered. 



All Vertebrates are in reality asymmetrical. The stomach projects 

 to the left of the middle line, while the heart and intestine show 

 spiral twisting and are asymmetrical in other ways. The asymmetry 

 of the gut and heart of the newt and frog has been experimentally 

 shown to be dependent on a factor situated in the gut-roof. At the 

 stage when the neural folds are still open, a square piece of pre- 

 sumptive neural tube material, together with the underlying gut- 

 roof, is removed from the dorsal side, about half-way down the 

 length of the embryo. The square piece is rotated through i8o° and 

 grafted back into place again so that the antero-posterior axis of the 

 piece is reversed. The result of such an experiment is a normal 

 embryo, except that it shows situs inversus of the asymmetrical 

 organs, i.e. the stomach is on the right and the intestine and heart 

 are twisted in the direction opposite to the normal. Rotation of the 

 presumptive neural tube material alone, without the underlying 

 gut-roof, does not interfere with the development of the normal 

 asymmetry. The ventral regions of the embryo are not touched by 

 the operation, and therefore the asymmetry of the heart and gut 

 must be determined by some factor or agency differentially dis- 

 tributed across the gut-roof^ (fig. 31). 



If, however, the square piece which in the previous experiments 

 was rotated, is simply removed, the embryo will show normal 

 asymmetry. This may mean either that the differential factor ex- 

 tends, though with diminished intensity, on either side of the gut- 

 ^ Pressler, 191 1; Meyer, 1913; Spemann, 1918. 



