DEVELOPMENT OF DIGITIFORM GLAND 351 



many as five secondary tubules. The same is true, in general, 

 of the secondary tubules, but those above the second order do 

 not show this last type of division. The tubules may be very 

 small or may be expanded at their peripheral ends. 



The anlage of the digitiform gland consists of a stratified 

 columnar epithelium of five or six layers of cells. This number 

 very soon becomes reduced to two in the central lumen and to 

 one or two in the tubules. Later, the epithelium of the central 

 lumen and that of the first part of some of the primary tubules 

 again becomes stratified. In the embryonic tubules beyond the 

 primary tubules the epithelium is simple columnar with oval 

 nuclei except at the growing peripheral ends where the cells are 

 irregularly round with spherical nuclei. 



In the adult these tubules contain three types of cells, (a) 

 cuboid cells with round nuclei, (b) columnar cells with oval 

 nuclei the long axis of which is at right angles to that of the 

 tubule and (c) flattened hexagonal cells with oval nuclei, the 

 long axis of which is parallel with the tubule. The peripheral 

 ends of the tubules are often pressed against the surrounding 

 tissue so that the cells there are forced into various shapes. The 

 central lumen is lined with an epithelium much like that of the 

 urinary bladder of higher vertebrates except that it contains 

 mucous goblet cells. The duct of the digitiform gland in the 

 adult has much the same structure as that of the intestine. 



Blood vessels are present in the digitiform gland from the 

 first. The digitiform artery arises from the dorsal aorta, an- 

 terior to the digitiform gland, and runs obliquely ventrally and 

 posteriorly in the mesentery to the anterior extremity of the 

 gland. It may or may not branch. It runs along the dorsal 

 side of the gland at the base of the mesentery, giving off lateral 

 rami which surround the gland. These may anastomose and 

 form a slender vessel on the ventral side. From these arteries 

 very numerous small capillaries enter the substance of the 

 gland, penetrate among the tubules, and form from two to four 

 longitudinal venous sinuses along the central lumen of the gland. 

 The ventral sinus is the larger and the others empty into it 

 toward the proximal end of the gland. This sinus extends along 



