DEVELOPMENT OF THE ELASMOBRANCH LIVER 347 



dorsally which deepens the groove between them and the gut 

 mesially. The pars ductus expands somewhat and becomes 

 more sharply marked off from the archenteron posteriorly. The 

 anterior median pouch also shares in this expansion but shows 

 no other changes. Likewise the gall bladder becomes rotund, 

 a distinct groove intervenes between its dorsal-anterior angle 

 and the liver anlage and a ventral notch of some depth separates 

 the sack from the anterior wall of the yolk-stalk behind. In 

 front of this ventral notch the sac is still continuous with the 

 hver anlage proper, but the longitudinal construction between 

 the two structures mentioned in the description of the preceding 

 embryo is present in a more distinct form. These changes are 

 illustrated in figures 35 and 36, of a wax reconstruction from an 

 embryo 9 mm. in length, the general anatomy of which has been 

 previously illustrated in graphic reconstruction in figure 1 1 of the 

 Normal plates of Acanthias. 



A little later, as shown in an embryo of sixty somites with three 

 open and two closed gill pouches and two complete turns of the 

 spiral valve, the lateral pouches lose their expanded outline, and 

 becoming flattened laterally, enter upon a decided dorsal growth 

 (figs. 37 and 38). At the same time their posterior margins 

 become sharply differentiated so that they extend out from the 

 gut at an abrupt angle and their distal edges show several slight 

 irregularities. The anterior median pouch remains practically 

 unchanged. The lateral grooves along which the liver eventually 

 separates from the fore gut above it are now considerably deep- 

 ened and extends the entire length of the line of attachment of 

 the liver evagination, although they are still shallow anteriorly. 

 The left lateral pouch bears on its lateral surface three small 

 longitudinal ridges. These together with the dorsal irregulari- 

 ties mentioned above constitute the anlagen of hepatic tubules 

 and will be discussed in the section dealing with these structures. 

 The gall bladder while no larger than in the preceding embryo is 

 separated from the anterior wall of the yolk-stalk by a deeper 

 ventral notch and the constriction between the sack and the 

 median part of the liver above is more pronounced. Both this 



