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. Theories 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.
