DEVELOPMENT OF SUPRAPERICARDIAL BODY 409 



structure between the ordinary branching gland and the closed 

 follicular or typical ductless gland. 



3. The early development of the gland is always limited to 

 the left side. The right gland is inconstant in its development 

 and does not appear until about the 33 mm. stage. When 

 present, the right gland has the form of a single cyst or solid 

 cord of cells which degenerate completely in the course of de- 

 velopment. 



4. The vesicles of the adult gland are derived from branching 

 tubules which develop from the expanded ventral portion of the 

 gland and the connecting-stalk. The process of formation of 

 the new tubules can, for convenience of description, be divided 

 into three stages: 



a. A small, but definite, localized proliferation of cells at the 

 site of the future tubule. 



b. An extension of the lumen of the parent tubule into the 

 new cell mass, the cells of which rotate so as to become placed 

 radially with respect to the new lumen. The newly-formed 

 outpouching may remain connected and grow en masse with 

 the parent tubule forming a branching tubule; or 



c. It may become constricted off by ingrowth of the epithelial 

 walls from one or both sides. The nuclei of the cells of the 

 ingrowth become placed tangentially with respect to the circum- 

 ference of the lumen, but when the tubule is completely sepa- 

 rated they rotate through an arc of 90° to assume a position 

 radial to the new lumen. 



5. Throughout the later development of the gland and in the 

 adult, the tubules and vesicles are closely surrounded by a 

 'rich' plexus of thin-walled veins which drain laterally into the 

 inferior jugular trunk. Accompanying the veins and imbedded 

 in some cases directly in their wall is a network of vessels which 

 I have interpreted as lymphatics. The lymphatics and veins 

 communicate freely and are analogous to the 'venae lymphati- 

 cae' and 'vasa lymphatica' of the thyreoid of selachians, as de- 

 scribed by Ferguson. 



6. The vesicles are lined by a single layer of narrow columnar 

 cells which are actively secreting mucus. The nuclei are elon- 



JOURNAL OF MORPHOLOGY, VOL. 28, NO. 2 



