1019 



This reino\al also reaches to the part of the gut situated before the 

 buccal aperture, as is evident from the above-mentioned (p. 1017) 

 difference in lenglh of the two portions of the thyroid gland, whose 

 lowei- (later left) portion reaches much further forward tlian its 

 upper (latei' right) portion (cf. my paper 1914, tig. 1, 2, 3 out 

 of the metamorphosis and Lankestkr and Willey 1890, fig. 1 of a 

 young larva with 3 open gill clefts). 



As the entodermsacs of Hatschek now arise opposite each 

 other just in front of the region of the thyroid gland the conclusion 

 is obvious that their autimery is only apparent, even if we pay 

 attention to their first appearance only, without considering their 

 subsequent fate: The left sac is removed forwards and ought moipho- 

 logically to lie behind the right sac, as is indeed the case later on. 



The last arguments of Sedgwick are the asymmetrical situation 

 of the anus and the olfactory pit. 



The removal of the olfactorj^ pit, which develops out of the 

 mesial ly situated neuroporus, to the left side is easily explained by 

 the spiral motion with rotation of the axis from right to left: The 

 olfactory pit had to catch up water and therefore to remove to 

 the left side. 



The asymmetrical situation of the anus could however not be 

 explained so easily. Not only does it lie on the left side in the 

 adult animal, but according to Hatschek (J 881) it is already situated 

 on that side in the young larva with one gill cleft. 



In the same way as the gill clefts serve for letting out the water 

 taken up by the mouth of the animal, the anus serves for letting 

 out the undigested food. The respiratory water as well as the faeces are 

 propelled not so much by muscular contractions as indeed princi- 

 pally if not exclusively by ciliary motion, thus by weak forces. 



To the same extent as it was profitable for the gill clefts to 

 remove to the right side one would also expect this in the anus. 

 It would have to be considered as a postulate of the theory that 

 the anus either originated in the mesial plane, to remove later on, 

 not to the left, but to the right side, or that it developed directly 

 on the right side. 



In the hope of being able to discover something in young larvae 

 that would throw some light on the question, I found beyond expec- 

 tation, that the anus in larvae with one gill cleft is situated not on 

 the left but ou the right side of the body. 



This was clear beyond the slightest doubt in series of cross sec- 

 tions. In preparations in toto it was however impossible, even with 

 the strongest dry lenses, to determine in such a young larva on 



