THE DEVELOPMENT OF PALUDINA VIVIPAKA. Ill 



externally at this stage, for not only is the symmetry Jis- 

 turbed by the inequality of the rudiments of the mantle 

 cavity just noticed_, but the whole visceral hump appears as if 

 slightly tilted. The apex lies somewhat to the left of the 

 vertical plane, which would divide the head and foot 

 symmetrically, while the mantle fold on the left of the body 

 is at a lower level from that on the right. This tilting is 

 difficult to represent in surface views, though by rolling the 

 whole embryo about it is perfectly easy to see. It is, how- 

 ever, sufficiently obvious in the transverse section through the 

 hump (fig. 11), which is orientated on the page as it would be 

 on the body, the line a a representing the vertical plane 

 through head and foot. 



In the internal organs there is little to add to von Erlauger's 

 (5) account. The stomach and liver together form, as before, 

 the apex of the visceral hump ; they are now well-defined, 

 though still retaining their wide communication with each 

 other. Posteriorly the liver lies distinctly to the left, while 

 further forward it gradually becomes almost ventral, passing- 

 over to the right, as before, in front of its opening into the 

 stomach (fig. C\). From the posterior end of the stomach 

 the rectum runs almost vertically downwards to open in the 

 position already noticed between the two rudiments of the 

 mantle cavity. Just dorsal to these, and anterior to the 

 rectum, are the two rudiments of the pericardium, still 

 separated from each other, that on the right being a good 

 deal the larger of the two, as we saw to be the case also 

 with the mantle cavity (fig. C\, fig. 11). At the left, in 

 fig. 11, a slight thickening of the pericardial epithelium is 

 seen, which must be the rudiment of the left kidney ; the 

 right kidney is not yet formed. In the head region the first 

 appearance of the radular sac is noticeable, and also the 

 appearance of the ganglia of the central nervous system as 

 thickenings of the ectoderm, as they have been already 

 described by von Erlanger. 



Stage D (fig. D, fig. 12). — The foot now begins to show 

 for the first time a tendency towards the formation of the 



