190 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



labyrinth of the ad alt gland is attained, and not by any system of coiling 

 of one or a few tubules. 



I have already (p. 180) called attention to the fact that in the embryo 

 the endsac is ventral to the ectodermic sac, whereas in the adult organ 

 the endsac is dorsal to the mass of ectodermic tubules (labyrinth) which 

 are developed from the ectodermic sac. How is this change of position 

 brought about ^ In the first larva — as is seen in Figures 33-35 (Plate 4) 

 and Figures 40, 44-48 (Plate 5) — the endsac lies chiefly ventral to the 

 ectodermic sac, but also extends in part dorsad up along the lateral face 

 of the latter. Sections of later larvae show that the chief region of 

 growth in the endsac is in this dorso-lateral projection, while in the 

 ectodermic sac the chief area of growth and complication by evugination 

 (see p. 189) is in the ventral region, by means of which it extends laterad 

 ventral to the endsac and between it and the ventral wall of the append- 

 age. There is, then, by this differential growth a rotation of the centre 

 of mass of each of these parts of the organ. This rotation is around the 

 antero-posterior axis of the gland, and, if one imagine himself viewing 

 this from an anterior po^nt, this rotation is clockwise in the right gland, 

 and the reverse in the left. There is no actual twisting, but only a 

 change in position of the centre of mass of endsac and of ectodermic 

 sac. In the larva 18 millimetres long this process has progressed so far 

 that the plane which separates endsac from ectodermic sac is approxi- 

 mately parasagittal in position. Since the antero-posterior extent of the 

 endsac is mucli less than that of the ectodermic sac, the former can grow 

 dorsad and then mediad without disturbing the stalk which connects the 

 evaginated vesicle with the ectodermic sac, because the endsac lies en- 

 tirely posterior to this stalk. This process also makes it easy to under- 

 stand how it comes about that the endsac in the adult gland empties 

 into the lateral anterior lobe of the labyrinth (see p. 164), but has no 

 direct connection with the median anterior lobe. 



I have not at my command the material to enable me to follow this 

 process of differential growth in the adolescent stages; but as far as I 

 have been able to examine the larvse, the conditions found agree with 

 the process which I have mentioned, and I believe that it is in this way 

 that the relative position of ectodermic sac and endsac as found in the 

 embr3'o comes to be reversed in the adult, where the endsac is dorsal to 

 the ectodermic sac or labyrinth. 



