of the Fishery Board for Scotland. 



275 



is noticed is posterior to the invagination (d.c.) beginning to extend into 

 the liver (lr.). If this widening is the rudiment of the stomach, then 

 either the invagination is not the ductus choledochus, or the opening 

 of this duct must shift further backward, so that the relative position of 

 stomach and opening of duct may become normal. That the invagination 

 is the beginning of the ductus choledochus is proved by a series of 

 sections in older embryos (cf. figs. 4, 5, 7, and 15, d.c), and no evidence 

 is afforded by examination of my sections that the unlikely alternative 

 of a change of relative position is true. I can only conclude, therefore, 

 that Lereboullet, as well as Vogt, is describing a condition that only 

 obtains when the liver is partially separated from the ventral wall of the 

 intestine ; certainly such a description does not hold good for the plaice. 

 Vogt's figure, 142, in Table V., showing the elargissement, seems to be 

 taken from an embryo even more advanced than that represented in fig. 4 

 of my plate. The invagination of the ductus choledochus, which in fig. 2 

 is shown as a slight depression in the wall of the intestine adjoining the 

 liver, becomes deeper as the liver is being separated from the intestinal 

 wall, so that in fig. 4 it extends in a curved manner into the substance 

 of the liver. While this has been taking place, the ventral protuberance 

 assumes a more pronounced form, and the liver pushes against the yelk- 

 sac underneath. The first differentiation of the organs — liver and intes- 

 tine — from one another, is indicated anteriorly (fig. 3) where the columnar 

 cells of the intestine are sharply marked off from the cells of the liver. 

 This division of liver and intestine anteriorly has extended in fig. 4 almost 

 as far as the opening of the duct. While the separation goes on at 

 first anteriorly, in less than two days a posterior separation also takes 

 place. The result of the whole is that the liver is now of a reniform 

 shape, with the hilum situated slightly posterior. 



When it has attained to this shape there is a further development in 

 the character of the digestive tube. The upper wall of the gut has under- 

 gone thickening, and projects into the lumen of the tube opposite the 

 opening of the ductus choledochus ; possibly this abutment represents 

 the beginning of the formation of the pyloric valve. The liver grows in 

 a dorsal direction on either side till it completely envelopes the intestine 

 (fig. 5, lr.), and it is only in the region where the duct is given off from the 

 tube that the cells of liver and intestine are indistinguishable. The 

 liver, which up to this time was uni-lobular, has now assumed a two-lobed 

 form, a left ventral, and a right dorsal lobe. The intestine in cross 

 section is tri-radiate at the point of opening of the ductus choledochus, 

 the lower ray being formed by the lumen of the duct. Still the liver is 

 an accumulation of cells comparable to Vogt's ' essaim d 'abeilles,' but soon 

 it assumes a lobulated form, and the right and left lobes are themselves 

 being divided into lobules (figs. 6, 7, 14, 15, 22, and 23). 



The compact nature of the liver does not continue. Spaces about this 

 time are seen in the substance of the liver, and represent hepatic ducts. 

 The large cavity situated between the right and left lobes (fig. 6, g.b.), is 

 the gall-bladder. Lereboullet's contention that .the gall-bladder has an 

 origin independent of the liver is only in a limited sense true. Speaking 

 of the pike, he says : — ' La vesicide biliaire par ait, d'apres cela, avoir une 

 origine independante du foie et se produire par exaertion de I'intestin.' 

 My sections show that the gall-bladder is not produced in the interior of 

 the liver, as, e.g., the auditory sac is produce^ by an independent separa- 

 tion of radially-arranged cells, but that it is formed by a widening of the 

 principal duct. Nevertheless, the cells of the wall of the bladder are 

 principally cells of the liver, which are used in the formation of the bladder- 

 wall. The ductus choledochus shifts its position gradually to the left side, 

 as illustrated in figs. 4, 15, 5, 6, 7 ; and after it has turned through an 



