102 ISABKLLA M. DUCMMOND. 



gonad {g.) is in reality the radimentnry loft kidney^ and tlint 

 he has missed the true origin of the gonad altogether. I 

 confess that I cannot fully understand this figure, but as far 

 as I can make out^ the gonad should lie in a direction at 

 right angles to that in which it is shown^ if it is to maintain 

 the relation to the liver shown in fig. 6, and again in fig. 17, 

 and this would bring it into about the right relation with the 

 evagination of the pericardium marked g., if we regard this 

 latter as the rudimentary kidney. This view, moreover, 

 would account for the discrepancy which exists between 

 von Erlanger's account of the origin of the gonad as an 

 evagination, and my own. The divergence in our descriptions 

 of the duct is not so easy to explain, but I feel sure that 

 von Erlanger is not correct when he speaks of the gonad 

 (the left kidney, according to the present view) growing 

 towards the ingrowth of the mantle cavity, for these are, 

 and have been from the first, in the closest connection with 

 each other (see figs. 2 and 15, l.Ji. and I.7)i.c.). 



Fig. 5 shows a further development. The pericardium is 

 cut across at the extreme right of the figure, from this the 

 left kidney passes downwards at I. li., and the communication 

 with the pericardium is still shown at r.p.c.aj). On the left 

 of the figure is seen the gonad {g.), still solid, but now fused 

 with the wall of the kidney, so that the little connecting 

 portion of thickened pericardial epithelium is no longer 

 distinguishable. The duct is cut at l.m. c, and does not yet 

 open into the kidney. The exact position of these organs in 

 the body may be seen in fig. 17, which is a drawing of a 

 closely adjacent section of the same series. The lettering is 

 identical, and the gonad, duct, and pericardium are all shown. 

 It will be seen that essentially the same relations obtain as in 

 earlier stages, the gonad following the livei-, and keeping 

 always on the inside of the coil. 



From this stage onwards very rapid growth of the gonad 

 takes place, so that it soon reaches the extreme tip of the 

 visceral hump, and then it takes part in cvoiy coil as it is 

 formed. At the same time it becomes hollowed out, from 



