THE DEVELOPMENT OF APLYSIA PUNCTATA. 517 



dorsal surface of the stomach, slightly to the left of the 

 middle line. The anus opens into the mantle cavity a short 

 distance below the secondary kidney. 



It will here be convenient to say a few words about the 

 torsion which the embryo undergoes in the course of its 

 development. This involves two processes, perfectly distinct 

 from one another, though in Aplysia they take place simul- 

 taneously, one being the oro-anal flexion, so characteristic 

 of molluscan organisation, and the other the rotation of the 

 anus and adjacent organs through more than 120°, round 

 an axis coinciding Avith the antero-posterior axis of the 

 embryo. By the foi-mer process the anus is carried forward 

 to open anteriorly, and the intestine to lie ventral and parallel 

 to the oesophagus; while by the latter, torsion, properly so- 

 called, the intestine, liver, kidney, and coelom are carried 

 from the ventral surface up on to the right side of the body. 

 This movement is mai-ked externally by the change in position 

 of the anal cells (PL 22, figs. 4, 5, 6, 7, 8). The shell alone 

 appeal's not to be affected by the torsion, for before this 

 process is complete it has in miniature nssumed its final 

 shape, which it retains, while the organs inside it are being 

 twisted in the manner described. 



The Liver. — The endomeres A and B which form the left 

 liver remain very large, and for a long time do not divide. 

 At a stage corresponding to fig. 5 the nuclei divide, and 

 thenceforward multiply slowly ; but for some time no corre- 

 sponding cytoplasmic divisions are to be distinguished. The 

 nuclei are at first large and filled with yolk, scattered amongst 

 irregular fragments of chromatin. Later they become reduced 

 in size after repeated divisions and lose their yolk contents. 

 The endomeres are full of yolk-granules, and their cytoplasmic 

 structure is thereby entii-ely obscured. Of the two, B is 

 approximately dorsal and A ventral. The effect of torsion is 

 to move B more over to the left and A slightly to the right. 

 They remain perfectly distinct from one another for some 

 time, but eventually become fused together on the left side 

 to form a single organ, the left liver. We have spoken 



VOL. 55, PART 3. NEW SERIES. 34 



