104 ISABELT,A M. DRUMMOND. 



of this account, to notice von Brlanger's description of the?e 

 organs when they have more nearly attained their adult 

 condition, and are beginning to show the development of 

 the actual genital cells. Such stages, he says, show '•' dass 

 bei beiden Geschlechtern ein wenn audi kurzes Stiick der 

 Leitungswege der Geschlechtsprodukte nus der Keimdriisen- 

 anlage selbst hervorgeht;" that is, this small region, ap- 

 parently belonging to and originating from the genital 

 organ itself, never, in either sex, gives rise to genital cells. 

 It is, of course, situated just at the junction of the gonad 

 and the duct, which, as he himself points out, '' findet in der 

 Gegend statt, wo der Verbindungskanal zwischen Herz- 

 beutel und Nieren sich findet." Surely this must be the 

 original left kidney, still distinguishable in the adult. 



To sum up, then, the original left kidney and its duct do 

 not, as von Erlanger believed, disappear. Their develop- 

 ment is arrested for a time, but they are both clearly present 

 at the time when the gonad is formed as a proliferation 

 from the original left dorsal extremity of the pericardium, 

 and from this time increase in importance. The gonad is 

 for a long time solid, and is connected with the kidney by a 

 thickening of the pericardial wall on the left side. At a 

 later stage the gonad becomes hollowed out, and its lumen 

 communicates with that of the original left kidney, pre- 

 sumably by means of the pericardial thickening, which must 

 also have become hollowed out. The genital products there- 

 fore pass through the original left kidney, and arc ejected 

 by its duct. 



The theoretical bearing of these conclusions is obvious, in 

 that they show how, even in the adult of one of the most 

 highly organised of the Khipidoglossa, an unexpectedly 

 primitive condition of the coclom and its derivatives still 

 obtains. Zoologists have long been agreed that the ancestors 

 of the Mollusca must have had paired gonads, which shed 

 their products into the coelom, to be carried thence by the 

 kidneys; that the coclom is now represented by the peri- 

 cardium, and that, though great modification has taken 



