266 RICHARD E. SCAMMON 



Most of these karyosomes are applied to the nuclear wall. The 

 hepatic cell retains the typical embryonic nuclear structure 

 much longer than do the cells of the mesenchyma, mesothelium, 

 nervous system or urogenital system. This typical embryonic 

 structure is lost first in the cells forming the gall bladder and 

 major hepatic ducts, next in the minor hepatic ducts which are 

 formed from cylinders already well started upon a development 

 towards typical hepatic parenchyma, and finally from the hepatic 

 cells proper. 



As has been remarked, fat droplets as indicated by vacuoles 

 in the protoplasm of the hepatic cells appear, when the embryos 

 obtain a length of about 25 mm. The use of special reagents 

 would no doubt demonstrate the presence of fat prior to this 

 stage. The droplets are found at first at the base of the cell, but 

 later, in embryos 65 to 95 mm. long, droplets are found scattered 

 through the entire cell body reducing the protoplasm to the 

 network which has been described for the adult hepatic cell of 

 selachians by Shore and Jones ('89) and Pilliet ('90). 



In Acanthias embryos about 30 mm. long the gall bladder 

 begins to press against the hepatic tissue which lies on either 

 side and above it. This process is probably brought about by 

 the great growth of the internal yolk-sac which lies below the 

 gall bladder. This pressure of the gall bladder causes some 

 degeneration of the hepatic tissue immediately surrounding it. 

 Toldt and Zuckerkandl ('78) have described a similar process in 

 the human embryo. 



5. Development of the hepatic tubules in other forms of selachians 



The development of the hepatic tubules in Torpedo, Raia 

 and Mustelus differs somewhat from that of Acanthias. In 

 these forms the lateral hepatic pouches do not reach the great 

 development found in Acanthias and tubules are formed from 

 these structures at a comparatively earlier stage. The omphalo- 

 mesenteric veins are somewhat larger than in Acanthias and at 

 the time when the individual hepatic tubules develop veinous 

 channels are found both medial and lateral to the hepatic pouches. 



