PLATE 7 



EXPLANATION OF FIGURES 



15, 16, 17 and 18 From four successive sections of a chick embryo of 7 days 

 (series no. 512; slide VII, sections 21 and 22; slide VIII, sections 1 and 2). Photo- 

 micrographs, X 250. 



In figure 15 four lymph spaces, in part lined with endothelium, are seen at the 

 left of the aorta (1). The one nearest the aorta, while in part lined with endothe- 

 lium, opens freely into the adjacent mesenchymal intercellular spaces. The larg- 

 est space also opens below in a similar manner. In figure 16, from the succeeding 

 section in this series, only one of the spaces appears, opening above into tissue 

 spaces. The others have simply become continuous with tissue spaces and there- 

 fore do not appear as distinct lacunae; this can be clearly demonstrated with the 

 microscope by changing the focus on the rather thick sections (20 micra). In 

 figure 17 the arrow points to the termination of the distinct lacuna of figure 16; 

 the free communication with the tissue spaces is quite obvious. In figure 18 there 

 are no lacunae, all those of the preceding sections having opened into the mesen- 

 chymal intercellular spaces. Aorta, 1; part of sympathetic nervous system, 24-- 



176 



