587 



Schultze (1894), and Morgan (1895). So far I have been unable to 

 obtain a sufficient number of newly laid frogs eggs to give this ex- 

 periment a fair test. 



Some interesting conclusions follow from this explanation of the 

 cause of inverse symmetry. It altogether does away with the idea of 

 a transposition of organs from one side of the body to the other 

 across the median plane, and it entirely disposes of the notion that 

 in cases of unpaired, asymmetrical organs, as for example the aortic 

 arch of man or the kidney of a gasteropod, we are dealing, in cases 

 of inverse symmetry, with the survival of the other member of a 

 primitively paired structure. In man it would be necessary to go 

 back to a stage which antedates the lowest mammals to find a con- 

 dition in which the right aortic arch is preserved, and in gasteropods 

 to the most primitive members of the phylum to find animals in 

 which both kidneys are present. To believe in such a tremendous 

 reversion requires more than ordinary faith and the unbelieving will 

 welcome a means of escape. If inverse symmetry is due to a rever- 

 sal of egg polarity, then the right aortic arch of an inverse man is 

 identical with the left arch of the ordinary man, and the kidneys of 

 Lymnaea and Physa are the same although on opposite sides of the 

 body in the two cases. 



Furthermore, some important conclusions follow as to the organi- 

 zation of the egg. If the reversal of polarity at the time of matu- 

 ration can bring about a total inversion of all parts of the embryo 

 and adult, then there must be a definite localization of germinal pri- 

 rnördia or anlagen in the egg before maturation, e. g., the sub- 

 stance out of which the kidney of the snail will ultimately form must 

 be definitely localized on one side of the chief axis, and so for every 

 other part. After the formation of both polar bodies and long before 

 the first cleavage the substance of the ectoderm and of the mes-ento- 

 derm can be distinguished with the greatest clearness in the eggs of 

 fresh water snails, a phenomenon which I shall describe more fully 

 in another place. Boveri (1901) and Fischel (1903), have recently 

 observed wholly similar phenomena in Echinoderm and Ctenophore 

 eggs, and may different persons have latterly been converted to the 

 view that the egg has a definite organization. No reasoning is neces- 

 sary to disprove the doctrine (now fortunately extinct) of the iso- 

 tropy of the egg, since the mere observation of almost any egg is 

 sufficient for that purpose, but if the inversion of the egg at the time 

 of maturation inverts the position of every part which develops from 

 it, no more convincing evidence could be found that "organbil- 



