DEVELOPMENT LYMPHATIC SYSTEM IN AMNIOTES 281 



The above, which is practically a literal transcript from Allen's 

 paper, fully covers the first point in the ontogeny of vertebrate 

 Ijrmphatico-venous hearts, i.e., the development of the lymphatico- 

 venous heart cavity in the following stages: 



a. Development of isolated mesenchymal spaces, and their 

 fusion to form the heart anlage. 



b. Establishment of a secondary connection between the heart 

 anlage and the adjacent caudal vein, by the breaking down of 

 the "mesenchjanal partition between the spaces and the caudal 

 vein." 



c. Accretion in the' centripetal direction of the ''more distal 

 isolated mesenchjanal spaces" to the ''mesenchymal cavit}^ al- 

 ready connected secondarily with the caudal vein." 



Allen here offers a very concise and clear picture of the devel- 

 opment of a lymphatico- venous heart. There is no 'budding' 

 from the caudal vein, nor any 'sprouting' from the 'venous 

 endothelium.' Heart-cavity and limiting border-cells develop 

 independently of the venous system, by union of the isolated 

 mesenchymal spaces and by modification of the boundary cells 

 {vide infra). 



Moreover, the heart thus formed establishes a secondary con- 

 nection with the adjacent caudal vein, while distaUy, new "iso- 

 lated mesench>Tiial spaces develop, increase in size, unite and 

 join the mesenchymal cavity already connected secondarily with 

 the caudal vein." 



This is progressive centripetal accretion of lymphatic anlages 

 as found everywhere in areas in wliich lymphatic development 

 is active. Allen further expresses himself clearly in reference 

 to the second and third points of morphological importance, 

 viz., the contents of the heart cavity above described, and the 

 genesis of the cells limiting the cavity: He states that "in places 

 the border mesenchymal cells are flattening to contribute to 

 the endothelium of the heart, while certain of the enclosed cells 

 increase in size, become spherical and differentiate into red cor- 

 puscles." Here we encounter, in the first place, the independent 

 development of lymphatic endothelium, lining the early mesen- 

 chymal lymphatic spaces. This endothelium has not 'sprouted' 



