HISTOGENESIS OF THE LIVER 251 



mm. in length (S.C. 14).^ This specimen bears two ridges on the 

 left hepatic pouch, but the right pouch is entirely smooth. The 

 irregularities of the margin of the hepatic pouches, which also 

 form tubules, are at first so ill defined as to be scarcely noticeable 

 unless reconstructed. Then it is seen that the formerly straight 

 dorsal margin has a wavy contour. 



The tubule ridges rapidly increase both in size and number. 

 New ridges appear ventral and mesial to the earlier ones and 

 extend to the base of the lateral pouches and upon the ventral 

 surface of the pars hepatica medialis of the median liver pouch. 

 No new ridges appear dorsal to the first ones and I think that no 

 new ridges arise between older ones. By the time the embryo 

 reaches a length of 10 to 12 mm. each lateral pouch wall and 

 lateral half Of the pars hepatica medialis bears seven to ten 

 tubule ridges. This is the total number formed on either side, 

 and when tubules thereafter arise from the pouch directly, they 

 do so as individual tubules and not in the form of tubule ridges. 

 Figure 18 is a view from the left side and below of a reconstruction 

 of the liver of an Acanthias embryo 10 mm. in length (S.C. 20) 

 showing the later form of the tubule ridges and the beginning 

 of the differentiation of tubules from them. 



Before all the tubule ridges are formed the ones which first 

 develop are broken up by transverse or oblique furrows into 

 rather irregular rows of low mound like elevations. These 

 elevations are the anlagen of the individual hepatic tubules and 

 may be seen in figure 18 referred to above. They are semi- 

 circular or nearly so in cross section and nearly twice as long as 

 broad, their greater length being always directed antero-poste- 

 riorly. Almost immediately the tubules begin an active outward 

 growth and each is differentiated into a distal extremity which 

 often is large and pouch like, and a more slender proximal stalk 

 which is connected with the hepatic pouch and which is circular 

 in cross section. The further growth of the tubules takes place 

 by the formation of tubules of the second order from the distal 



* In designating embryos the following abbreviations will be employed: H.E.C., 

 Harvard Embryological Collection; K.U.E.C., Embryological Collection, De- 

 partment of Zoology, University of Kansas; S.C, author's collection. 



