DEVELOPMENT OF THE ELASMOBRANCH LIVER 345 



broad and shallow and can be followed with difficulty above the 

 lateral diverticula. 



Two marked changes now take place in the liver anlage, bring- 

 ing about the condition described by Brachet ('96) and others as 

 the primitive one. These are the appearance of the gall bladder 

 and the distinct lateral and dorsal evagination of a part of the 

 lateral hepatic diverticula. An embryo 7 mm. in length (H.E.C. 

 752), which is probably a little younger than No. 21 of the Normal 

 plate series and a little older than Balfour's Stage H is the earliest 

 specimen in which I have observed any indica^tion of the gall 

 bladder. Both the separated posterior parts and the anterior 

 median pouch, formed by the fusion of the anterior ends of the 

 lateral hepatic diverticula, are more pronounced than in the em- 

 bryo just described. The anterior wall of the yolk-stalk is much 

 thickened over its entire extent, but particularly just below the 

 point where it becomes continuous with the floor of the fore gut. 

 The gall bladder is represented by a very shallow median depres- 

 sion at this place. Numerous mitoses indicate that the epithe- 

 lium is growing rapidly in this region. The early evagination of 

 the gall bladder must take place with some rapidity, as it is verj^ 

 difficult to find any specimens between the stages when this 

 structure is entirely absent and when it is a deep, well-marked 

 pouch. 



A specimen which illustrates both the early development of the 

 gall bladder and the growth of the lateral diverticula, and is 

 (juite comparable with the first members of Hammar's ('93) 

 and of Brachet's ('96) series of models, has been reconstructed 

 and figures 33 and 34 are anterior and left lateral views of the 

 model. This specimen, which is 7.5 mm. in length (H.E.C. 

 1503 and No. 24 of the Normal plate series), has 54 to 55 trunk 

 segments and four gill pouches, two of which open exteriorl}^ 

 The spiral valve makes one and one-third turns of the gut. The 

 distance from the last (fourth) gill pouch to the anterior wall of 

 the yolk-stalk is approximately one-fourth of the complete length 

 of the alimentary canal and about equal to the antero-posterior 

 diameter of the yolk-stalk. The lateral hepatic diverticula are 

 differentiated into three parts. Anteriorly they are fused, form- 



