76 ORIGIN OF POLARITY, SYMMETRY, AND ASYMMETRY 



tract forks at a point between the stomach and the cloaca, the right- 

 hand member frequently shows situs inversus, while the left-hand 

 member is normal^ (fig. 32). 



The rudiment of the heart can be divided in amphibian embryos 

 (at the tail-bud stage) by means of a longitudinal cut in the middle 

 line; each half rudiment will give rise to a heart, and while the 

 asymmetry of the left one is normal, that of the right one is 

 reversed^ (fig. 115). 



The remarkable point about these experiments and observations 

 is the constancy of normal asymmetry in the left-hand member, 

 and the restriction of situs inversus to the right-hand member. This 

 fact emerges still more clearly from those experiments in which the 

 blastula of the newt is constricted by a hair in the plane of bilateral 

 symmetry, and the hair is pulled tight, thus resulting in the com- 

 plete separation of two half-blastulae, of which one represents the 

 right and the other the left half of the original embryo. The left 

 halves develop into perfect little newts with normal asymmetry ; of 

 the right halves, about equal numbers show normal asymmetry and 

 situs inversus respectively.^ 



Whatever the asymmetry factor may be, it cannot be regarded as 

 an absolute and localised producer of one specific type of asym- 

 metrical structure — at least, not during the earliest stages of de- 

 velopment — and for the following reasons. It is true that when 

 newt embryos are divided into left and right halves at the blastula 

 stage, about half of the right-hand portions show reversed asym- 

 metry. But if the left and right blastomeres are separated from one 

 another (likewise by constricting in the plane of bilateral symmetry 

 with a hair) at the 2-cell stage, the right-hand blastomeres do not 

 show any greater tendency to production of reversed asymmetry 

 than is found to be the case in normal development of newts' eggs 

 — 2 to 3 per cent.* At the 2-cell stage, therefore, the asymmetry 

 factor has not become predominant on the left side at the expense 

 of the right. The same conclusion emerges from the simple ex- 

 periment of reversing an egg and forcing it to continue its develop- 

 ment in that position. If the prepotent normal-asymmetry factor 

 were definitely located on the left side at this stage, then since the 



1 Stockard, 1921 ; Morrill, 1919; Swett, 1921. - Ekman, 1924, 1925. 

 3 Ruud and Spemann, 1923. * Mangold, 1921 b. 



