1017 



He also gives his reasons: "for no one, so far as we know, has 

 ever attempted to bring the extraordinary features in the development 

 of the gill clefts, of the endostyle, of the head-cavities, the asym- 

 metric position of the anus and olfactory pit, into relation with the 

 asj-mmetrj of the mouth. The thing cannot be done. There is no 

 sort of connexion between these various asymmetries. They seem 

 to occur without rhyme or reason". 



Of all these arguments there is only one that seems to be consistent 

 viz. that concerning the anus. We shall consider them consecutively. 



An explanation for the removal of the gill-clefts to the right side 

 has just been discussed. The thyroid gland, indicated by Sedgwick 

 by the antiquated name of endostyle, is a mesial oi-gan in all the 

 Chordata. In Amphioxus it must also originate in the morphologically 

 ventral median line of the gut. We saw that this line has been shifted 

 to the right side in the branchial region of the larva. In the lai-va with 

 three open gill-clefts (Lankestek and Wili.ey, 1890, fig. 1) it runs over 

 the topographically dorsal (but morphologically ventral) edge of the 

 gill-clefts. If we continue it slightly rostrally it cuts the angle in 

 which the two portions (the later right and left halves) of the thyroid 

 gland meet. In the larva the thyroid gland thus lies topographically 

 asymmetrically; morphologically however symmetrically. 



The symmetry is in so far imperfect in that the lower portion 

 (the later left half) is longer and reaches considerably further rostrally 

 than the upper" portion (the later right half). We shall make use of 

 this difference later on in refuting another argument of Sedgwick. 



That the morphological median line of the gut was shifted to the 

 right side also anteriorly to the gill-clefts viz. in the neighbourhood of 

 the mouth can easily be explained by the considerable enlargement of 

 the mouth, which takes place not only in the longitudinal, but also 

 in the dorso-ventral direction. 



In the latter direction its lower border all but reaches the topo- 

 graphically xentral median line, which it even passes during the 

 metamorphosis, so that this typically left-sided organ is partly found, 

 not temporalli/ like the first gill-clefts, but permanently on the right 

 side of the animal. 



The thyroid gland cannot take its permanent place at the ventral 

 border of the gut until during the metamorphosis, after the gigantic 

 larval mouth has been reduced, temporarily even nearly to nil. 

 Then the difference in length between its left and right halves has 

 also disappeared. 



Sedgwick's third aigument are the extraordinary phenomena in the 

 development of the "liead-ca\ities" i.e. of the portions oflhecoelom 



66* 



