82 Papers from the Marine Biological Laboratory at Tortugas. 



In a specimen about 15 cm. in length, they measure about 6 mm. in 

 length and 4 mm. in breadth in their widest part. They occupy the 

 angle formed by the junction of the common coccygeal with a branch 

 of the posterior renal advehent vein of each side (fig. i). They do not 

 lie exactly parallel with the long axis of the body, but diverge laterally 

 at their anterior ends. The central cavity is broken up, more or less, 

 into communicating alveoli, especially in the posterior half. The walls 

 are made up of loose connective tissue and striated muscle fibers. The 

 hearts open into a vein connecting the common coccygeal with the 

 posterior renal advehent vein, or sometimes directly into the common 

 coccygeal vein. In the specimens studied, the rate of pulsation, shortly 

 after the animal had been chloroformed, was about four times per minute. 



A more detailed description of the anatomy of the lymph hearts, 

 together with their histological structure, is reserved for a later paper 

 on the complete anatomy of the lymphatic system of the loggerhead 

 turtle. 



DEVELOPMENT OF THE POSTERIOR LYMPH HEARTS. 



The first definite indications of the posterior lymph hearts appear 

 toward the close of the third week of development. But as early as the 

 end of the second week a noticeable vacuolation of the mesenchyme 

 takes place in the region immediately caudad of the anlagen of the pos- 

 terior limbs, giving it a spongy appearance. About the beginning of the 



fourteenth day this spongy 

 area becomes invaded by 

 small capillaries from the 

 first three dorsolateral 

 branches of the caudal por- 

 tion of the postcardinal 

 veins. These minute capil- 

 laries can be readily distin- 

 guished from the mesenchy- 

 mal spaces among which 

 they meander, by their end- 

 othelial walls. The increased 

 activity of the mesenchyme 

 cells due to the rapid growth 

 of the posterior limbs causes 

 a greater accumulation of 

 lymph in the intercellular 

 spaces, causing them to en- 

 large and become confluent 

 with each other and, to some 

 extent, with the invading capillaries. The exact relation of the tissue 

 spaces to the capillaries is very difficult to determine. However, after 

 studying a large number of thin sections taken through this region, 

 under the oil-immersion lens, I feel sure that these spaces do open 

 directly into the capillaries. 



Fig. 2. — Transverse section through lymph heart region of 

 Loggerhead Turtle embryo No. 230. (20 days, 14 mm.) 

 Xioo. a, veno-lymphatic channels; fc, vein; c, postcar- 

 dinal vein; d, aorta; e, sympathetic system; f, nerve. 



