292 Miss I. M. Drummond. 



the body. He describes it as formed in the same manner as the 

 definitive kidney, that is, as an evagination of the pericardial wall. 

 According to him, at a fairly early stage it degenerates, together with 

 the ingrowth of the mantle cavity, which formed its rudimentary duct, 

 and a later evagination of the pericardial wall in an almost identical 

 position gives rise to the gonad. This soon loses its connection Avith 

 the pericardium and becomes vesicular, while an ingrowth of the mantle 

 cavity, presumably the arrested kidney duct, grows towards it and 

 finally fuses with it to form the gonaduct. 



With the earliest stages of development as thus described the present 

 researches are in complete harmony. At a time, however, when, 

 according to von Erlanger, the rudimentary left kidney should have 

 completely disappeared, it is found to be still present, and to show 

 the normal relations both to the pericardium and the duct. No ten- 

 dency to retrogressive development could be found in either the left 

 kidney itself or in its duct. The development of the gonad also is not 

 what von Erlanger describes it to be, for, whereas the kidney is an 

 evagination of the original ventral wall of the pericardium, the gonad 

 arises as a solid proliferation of the dorsal wall, and is connected with 

 the kidney by a thickened ridge of pericardial epithelium. The 

 originally solid gonad^becomes secondarily hollowed out, and its lumen 

 communicates with that of the kidney by means of this thickening, 

 which also acquires a lumen. 



In the adult, therefore, the genital organs may be considered as 

 consisting of four distinct parts : (1) the gonad itself ; (2) a specialised 

 portion of the ccelom represented by the lumen of the pericardial 

 ridge ; (3) the kidney of the adult right side, through which the genital 

 products must pass to reach (4) the ureter of the definitive right side, 

 transformed into a gonaduct. These parts are shown in the figure, 

 which represents a longitudinal section through the genital organs before 

 maturity is reached. The long tubular gonad is seen at g. The second 

 division is indistinguishable at so late a stage, but must be represented 

 by the proximal portion of the gonad, and opens into the kidney (/. k) 

 at r. g. ap. The reno-pericardial aperture (r.p.c. ap.) is still open, but 

 will close before maturity. The duct is seen at /. m. c. 



II. Tlwrnies of Torsion. 



There are two main schools of thought which have to be considered 

 that of which Biitschli and, more recently, Plate are the chief expo- 

 nents, and that which is upheld by Pelseneer, Amaudrut, and Boutan. 

 Whereas the former uphold that unequal growth of the left side of the 

 body is an efficient ontogenetic cause of the torsion of the Gasteropoda, 

 the latter school believe that an actual twist of the body upon the head 

 through an angle of 180 takes place in the course of development. 



