TTTE DEVELOPMENT OE PAT.ITDTNA VTYIPARA. 127 



page in the same way with regard to head and foot, shows 

 clearly how stomach, liver, pericardium, kidneys, and mantle 

 cavity have all rotated in a perfectly definite manner, while 

 retaining unaltered their relations inter se, and explains 

 both the original dorsal and later ventral position of the 

 gonad, without the intervention of any dorsally growing liver 

 lobe. It is, moreover, striking that the torsion of the visceral 

 connectives and the apparent twist of the oesophagus noticed 

 on p. 119, keep pace perfectly with this rotation, so that it is 

 almost impossible not to connect the two phenomena. 



A comparison of figs. 16 and 17, on the other hand, causes 

 some little diflficulty, for although torsion seems to have taken 

 place through an angle of very nearly 180° in fig. 16, there 

 seems to be an apparent twist of about 90° further in fig. 17, 

 and so it may seem that we have proved too much by this 

 comparison of transverse sections. Two facts are, however, 

 noticeable. In the first place, whereas up to Stage G, a 

 transverse section of which is shown in fig. 16, the corre- 

 spondence is perfect between the degree of torsion shown by 

 the oesophagus and connectives on the one hand, and the 

 rotation of the organs on the other, this is no longer the 

 case in Stage H, where the connectives are only twisted 

 through 180°, while the organs in the posterior region of the 

 visceral hump have apparently rotated through about 270°. 

 It is indeed true that an accessory twist has been noticed 

 for the connectives as well as for the other organs in Stage H, 

 but this is clearly marked off from the regular twist corre- 

 sponding to that of earlier stages, which takes place more 

 anteriorly; and a study of the development of the mantle 

 cavity, and its disposition in this stage, makes it clear that 

 the two twists are quite unconnected. The visceral commis- 

 sure is formed from the floor of the mantle cavity, and, as the 

 original right portion of the latter is can-ied far down on the 

 left side of the body, so also is the origin of the corresponding 

 half of the visceral commissure ; while, as the original left 

 side of the mantle cavity remains feebly developed, it, and 

 consequently also the original left portion of the commissure, 



