364 RICHARD E. SCAMMON 



also farther separated from the posterior part of the median 

 pouch than is the right. 



The grouping of the tubules which arise from the lateral pouches 

 is quite distinct except for the left medial and anterior left 

 groups which have been rendered confluent by the vascular 

 changes just discussed. The dorsal and ventral posterior clus- 

 ters of the left side (fig. 45) are separate and the dorsal cluster 

 which throughout early stages precedes the ventral one in develop- 

 ment, is raised from the pouch surface and connected with it by 

 a short broad pedicle. On the right side (fig. 46), the anterior 

 right tubule group lies close to but is not fused with the right 

 medial one. As on the opposite side, both dorsal and ventral 

 clusters are distinct, and both are beginning to develop pedicles. 



These differences between the right and left parts of the pars 

 hepatica mediana and between the lateral pouches were present 

 to some small degree in the preceding stage, but are more notice- 

 able in this specimen and become more marked during later 

 development. They are due primarily to the unequal size of the 

 omphalo-mesenteric veins. 



It is well known from the studies of Rabl ('92), Mayer ('89), 

 Hammar ('93), and others, that in selachians there are at first 

 two omphalo-mesenteric veins of almost equal size. Early in 

 development however the right omphalo-mesenteric vein loses 

 its connection with the area vasculosa and for a time ends blindly 

 on the lateral wall of the yolk-stalk. Later a connection is formed 

 between the posterior end of the right omphalo-mesenteric vein 

 and the subintestinal vein by means of a channel lying to the 

 right of the pancreas. During the period while the right omphalo- 

 mesenteric vein ends blindly behind the blood from the yolk 

 sac and from the subintestinal vein passes forward through the 

 left omphalo-mesenteric vein alone, and this vessel in conse- 

 quence becomes much enlarged. At this time the rotation of the 

 gut from left to right about a longitudinal axis is in progress and 

 the passageway for the vascular channel between the lateral 

 hepatic pouch and the median hepatic pouch and fore gut is some- 

 what larger on the left side than on the right. This condition 

 is shown both in the anterior view of the model of an embi-yo 



