152 ADAM M. MILLER 



plexus in the loggerhead turtle (Stromsten), and with the pre- 

 cardinal division of the thoracic duct in the pig (Kampmeier). 



In the mesenchyme between and dorsal to the aortic roots 

 and arches in this embryo (fig. 24) a number of lymph spaces 

 have developed. Some of these lie within the masses of develop- 

 ing blood cells in this region {16a) while others are situated at 

 some distance from them. A few have coalesced to form a distinct 

 channel (19). The subsequent history of these lymphatics will 

 be given in the discussion of later stages. 



A few isolated lymph spaces are also found in and near the 

 root of the dorsal mesentery at the level where the celiac artery 

 enters the mesentery (figs. 24 and 25, 21). These belong to the 

 category of mesenteric lymphatics, but do not yet communicate 

 with the developing thoracic duct. 



The masses of developing blood cells in this embryo are exten- 

 sive (figs. 24 and 25, 16, 16a). Those associated with the main 

 portion of the developing thoracic duct lie either in the meshes 

 of the lymph plexus (17) or are already included within the vessels 

 composing the plexus. A plate-like mass extends over the mesial 

 aspect of the pleural cavity on the right side. The blood cells 

 still, as in preceding stages, tend to cluster about the celiac artery 

 (5). There is no connection in this stage between the masses 

 just mentioned and those which lie farther forward along the 

 mesial aspect of the great veins (10, 12), and which are associated 

 with the thoracic duct 'approach' of the jugular lymph sac (15a). 

 In the region between and dorsal to the dorsal aortic roots (2) 

 and aortic arches (3) there is seen a large mass (16a) with which 

 certain lymphatics (19), previously referred to, are associated. 

 Situated farther caudad are also several masses associated with 

 the lymphatics which lie along the dorso-lateral aspect of the 

 aorta (fig. 25, 16a). 



A feature shown in this reconstruction, and not in any other, is 

 a portion of the splanchnic plexus of veins (figs. 24 and 25, IJf).^ 



' Splanchnic venous plexus is the name given by Dr. A. J. Brown, in his yet 

 unpublished work on the development of the pulmonary veins, to the network 

 of venous capillaries in the wall of the alimentary tube in the earlier stages of 

 development. 



