DEVELOPMENT OF SUPftAPERICARDIAL BODY 399 



increased both in size and number and in some places are in 

 close proximity to the walls of the tubules. In the pharyngo- 

 pericardial wall just caudal to the gland, the vessels form a 

 coarse-meshed plexus which communicates laterally with the 

 inferior jugular vein and caudally with the common cardinal vein. 



From the 47.3 mm. to the 95 mm. stage the veins surrounding 

 the growing tubules increase greatly in number and form a close- 

 meshed plexus which completely envelops the gland except on 

 its ventral surface (fig. 7). The vessels of the plexus have very 

 thin walls, and in most cases they are separated from the gland 

 tubules only by the membrana propria. 



A number of small arterial twigs ramify in the mesenchyma 

 between the tubules. They are branches from a larger bilateral 

 trunk in the pharyngo-pericardial wall, just lateral to the cardio- 

 branchial cartilage and in close proximity to the gland. It 

 arises • cranially from the sinus formed by the median hypo- 

 branchial artery and terminates posteriorly by dividing into 

 the pharyngeal and gastric arteries. 



In the 'pup' stage (21 cm. long) the veins surrounding the 

 gland tubules are larger, but relatively less numerous than those 

 found at 95 mm. Although the mesenchyma of the earlier 

 stages has at this time become converted into a fine-fibered con- 

 nective tissue, the walls of the veins are still very thin. In many 

 places, however, the walls- of the vessels are invaginated by 

 small nodules of lymphocytes in the center of which is found 

 an arteriole. 



Accompanying the veins, and in most cases actually embedded 

 in their walls are numerous clefts or channels which I have 

 interpreted as lymphatics. These channels, many of which lie 

 in close proximity to the walls of the gland vesicles, communi- 

 cate freely with the vessels previously described as veins (fig. 

 18). The arrangement of the vessels here is very similar to 

 that described by Ferguson ('11) in the thyreoid of elasmo- 

 branchs. Ferguson, using the terminology of Favaro, calls the 

 veins 'venae lymphaticae,' and the lymphatics 'vasa lymphatica.' 

 The content of these vessels in the thyreoid, Ferguson believes, 

 depends upon the blood pressure, which is determined by the 



