1016 



lie oil the topographical right side, thej morphologically still belong 

 to the left side. It is first in the following period, that of metamor- 

 phosis, that the right side of the pharynx also grows in bi-eadth 

 and gradually pushes the left side, with the gill-olefts back to the side 

 where it belongs. 



The right gill-clefts now arise for the first time, liaving earlier 

 been hindered in their appearance by the crowding of the clefts ot 

 the left side. 



With a screw-like motion turning from right to left such an 

 occurrence of the gill-clefts is no longer odd. The secondary moudi 

 of the larva takes up water that flows away through the gill-clefts 

 by means of ciliary motion. The gill-clefts of the left side had now 

 to remove^) to the topographical mesial line or better still to the 

 topographical right side to prevent their taking in water. 



In my paper of 1914 I mentioned already that the form of 

 motion of the young larva can explain not only the origin of the 

 mouth on the left side, but also the odd occurrence of the gill-slits. 

 It was also mentioned that such a form of motion moreover makes 

 it comprehensible that the organ of hearing (organ of equilibrium) 

 could be lost, because the axis of a rotating object if the velocity 

 be sufficient, is stable; while it cannot be expected in such a form 

 of motion that the eye would be developed to a complicated organ, 

 forming images. 



At that time the criticism of Adam Sedgwick on my explanation 

 of the asymmetric location of the mouth, in the third part of his 

 text-book, published in 1909, was unknown to me. It is hidden in 

 the chapter on the Echinodermata, in which the mouth originates symme- 

 trically in the ventral median line, to remove later on to the left side. 



Admitting that this case of secondary asymmetry differs from the 

 phenomenon in Amphioxus, where the mouth is developed piimarily 

 asymmetrically Sedgwick says that Amphioxus and the Echino- 

 dermata still have the left-sided location of the mouth in common. 

 He continues (I.e. p. 162) "Here again we have a character which 

 strikes us fro'm its very rarity, for it is found in no other Coelomate 

 nor so far as we know in any other member of the animal king- 

 dom. It also strikes us by its strangeness and inexplicableness. In 

 Amphioxus no serious attempt has been made to explain it". 



My explanation was thus according to Sedgwick no serious attempt. 



1) The removal of a mesially situated organ to the left side is not without 

 analogy ; it occurs e.g. in the heart and stomach of man. As the nerves of these 

 organs arise from both halves of the body it is also clear, without knowing their 

 development, that they must have been situated mesially. 



