DEVELOPMENT OF DIGITIFORM GLAND 343 



The duct extends some little distance into the proximal por- 

 tion of the gland, running at first in the ventral half but more 

 distally, taking the central position. A section through this 

 region shows a structure, part gland and part duct, each with 

 its characteristic epithelium. The epithelium of the duct near 

 the intestine is of the stratified columnar type, containing, on 

 the free border, very numerous goblet cells which gradually 

 diminish in number toward the gland where only occasional 

 goblet cells are found. The external layers of the intestine, 

 namely, the serosa, outer longitudinal smooth muscle, inner circu- 

 lar smooth muscle and submucosa are continuous with corre- 

 sponding layers of the duct of the digitiform gland at the junc- 

 tion of the duct and intestine, but further distally the layers 

 of muscle gradually break up, first the longitudinal and then 

 the circular layer, so that only small bundles of fibers and iso- 

 lated cells are seen, scattered through the connective tissue. 



Development of the blood vessels. When the anlage of the gland 

 moves laterally it carries with it the blood capillaries in the wall 

 of the gut (figs. 1 and 2). These are supplied through capil- 

 laries in the mesentery and are seen in all the stages studied 

 (figs. 1 to 8). They arise from the aorta and form the digiti- 

 form artery. The capillaries in the gland penetrate among the 

 forming tubules to the wall of the central lumen. Here they 

 gradually form large longitudinal venous sinuses, as seen in the 

 figures referred to above. In early stages (figs. 4, 5, and 8) 

 the circulation seems to be of the sinusoidal type wherein large 

 peripheral sinuses connect with centrally located sinuses by rela- 

 tively large channels. This is the usual type of circulation found 

 in the glands of Acanthias embryos. The sinusoidal circulation 

 in the digitiform gland is lost, however, in its later development 

 and there is present instead a peripheral arterial supply joined 

 by capillaries to a central venous drainage system. 



Development of the mesentery. In early stages the dorsal mes- 

 entery of the hind gut is attached along the intestine, from the 

 spiral valve to the cloaca. As the digitiform gland develops 

 and pushes laterally from the intestine (15 to 23 mm. embryos), 

 it carries with it the intestinal mesentery, thus leaving a portion 



