352 E. R. HOSKINS 



the duct of the gland as the digitiform vein and empties into 

 the dorsal intestinal vein. The circulation is sinusoidal at first. 

 Later, capillaries are found instead of sinusoides. 



Evidences of degeneration are present in the adult digitiform 

 gland. 



THE POST-VALVULAR SEGMENT OF THE INTESTINE 

 Introduction 



Since comparative anatomists are not in agreement as to the 

 terms to be applied to the different parts of the intestine between 

 the spiral valve and the cloaca it seems proper to refer to this 

 region of the intestine as post-valvular. As numerous descrip- 

 tions of the intestine of young embryos already exist (Scammon 

 '11), only the later stages need be considered here. Only that 

 part of the intestine which develops from entoderm will be 

 described in detail. 



So far as the available literature shows, there is no descrip- 

 tion of the development of the post-valvular portion of the 

 intestine in later stages of selachian embryos. 



As to the embryonic occlusion of the digestive tube in these 

 forms, several observations have been made. Balfour ('78) 

 stated that in embryos of elasmobranchs the oesophagus is solid 

 for a time. Blanchard ('78 b) mentioned that he found the 

 gut to be solid in the region of the digitiform gland in an Acan- 

 thias embryo 23 mm. in length. Scammon ('11) gives the length 

 of the solid portion of the oesophagus and hind gut in Squalus 

 acanthias embryos in several stages. He noted the longitudi- 

 nal grooves on the surface of the gut in this region. 



It is well known that atresia of the intestine occurs normally 

 in higher animals also, as for examples, in the hind gut of the 

 chick (Minot '00) and in the human duodenum (Trandler '00, 

 Johnson '10). 



Development of the external form 



In an Acanthias embryo of 15 mm. (fig. 20) the intestine, 

 posterior to the spiral valve is somewhat cylindrical in form, 



