THE LATER DEVELOPMENT OF THE CHICK 349 



heart. This arrangement continues until this embryonic 

 kidney is replaced by the definitive kidney of the adult (meta- 

 nephros), when the posterior cardinal veins, then the renal 

 veins, connect directly with the subcardinal veins and the vena 

 cava, and the renal portal system disappears. The subclavian 

 veins which originally opened into the proximal parts of the 

 posterior cardinals, acquire openings into the ductus Cuvieri 

 near the vertebral and external jugular veins. 



4. The Lymphatic System and Spleen 



The development of the lymphatic system is imperfectly 

 known in the chick. A pair of anterior or cervical lymph hearts 

 appears during the fifth day, and. from these lymphatic net- 

 works grow out extending posteriorly, parallel with the lateral 

 veins of the body; by the eighth day these nets are developed 

 into definite lymphatic vessels. A pair of posterior or caudal 

 lymph hearts appears early the seventh day as a series of lat- 

 eral outgrowths from the first five coccygeal veins (tributaries 

 of the posterior cardinal veins). These branches anastomose 

 with one another, forming an irregular sac or lymph heart, on 

 each side, which remains connected with the second, third, and 

 fourth coccygeal veins. The walls of the hearts become muscu- 

 lar and rhythmically contractile during the eighth and ninth 

 days. 



During the ninth day the posterior lymph hearts connect 

 with a system of lymphatic spaces around the posterior section 

 of the dorsal aorta, and this space connects in turn with the 

 thoracic ducts. These lymphatic trunks are visible on the eighth 

 day, when they are described as two solid strands of mesen- 

 chyme, extending from the thyroid body to the roots of the 

 coeliac artery. They become hollowed out and connect with 

 the ductus Botalli, the dorsal aorta and the ductus Cuvieri. 

 These canals then approach and fuse, and about the twelfth 

 day connect with the posterior lymph hearts by way of the 

 lymphatic vessel around the posterior dorsal aorta. (It is 

 entirely probable that the connections between the thoracic 



