DEVELOPMENT OF DIGITIFORM GLAND 345 



9). The anterior end of the gland extends beyond the mesen- 

 tery for a short distance in most of the older specimens studied. 

 In the adult the anterior third of the gland may lie free in the 

 body cavity. 



Histology of the adult gland 



The histological structure of the digitiform gland of the adult 

 selachian has been described in a large measure by various 

 writers, hence it is unnecessary to give a lengthy description 

 here (see Introduction, and Disselhorst '04). Attention may 

 be called however to the accompanying figures 10, 11 and 12, 

 which show the microscopic appearance of this peculiar gland. 



As shown in figure 10, the gland is nearly circular in section. 

 It consists of thousands of radial tubules grouped around a large 

 irregular central lumen and surrounded by a narrow capsule of 

 fibrous tissue and peritoneum. The sections shown in the fig- 

 ures 10, 11, 12, are from the middle segment of the gland. The 

 digitiform artery lies at the base of the mesentery. Near the 

 lumen, projecting into it, but under the epithelium, may be 

 noted the ventral longitudinal venous sinus or digitiform vein. 

 Between the gland proper and the capsule, in some sections, is 

 a small group of isolated tubules which join the tubules of the 

 gland proper at a different level. The capsule is composed of a 

 serous outer covering of simple epithelium and an underlying 

 connective fibro-muscular layer. The connective tissue is mostly 

 of the white fibrous type. Among these fibers are both circular 

 and longitudinal smooth muscle fibers but they are not in defi- 

 nite bands at this level. In other sections the muscle is seen in 

 large bundles, the circular internal to the longitudinal. There 

 are numerous nerves and capillaries in the middle of the capsule. 

 From the capsule a septum of connective tissue enters the paren- 

 chyma. 



The parenchyma consists of tubules surrounded by connec- 

 tive tissue and capillaries (figs. 11 and 12) as has been men- 

 tioned above. The cytoplasm is very granular. The granules 

 are in some places arranged in rows. In the cytoplasm near 



JOURNAL OF MORPHOLOGY, VOL. 28, NO. 2 



