454 SUHIMAKY OF GUUUKNT UESEAKGHES KELATING TO 



hepatic sinusoids and mesonephric glomeruli of this stage also show 

 extensive hemopoietic capacity. 



Giant cells, both mono- and polynuclear, are abundantly present in 

 the yolk-sac only at about the lo'mm. stage of development. They 

 may arise from ' endothelium or directly from heemoblasts. They are 

 giant ha^moblasts, and apparently function as multiple erythroblasts in 

 which normoblasts differentiate intracellularly. The several stages in 

 haemopoiesis, represented successively by hgemoblasts, erythroblasts,. 

 and normoblasts, with transition stages, are abundantly present in the 

 yolk-sac of embryos from 5 to 15 mm. 



Cell - clusters in Dorsal Aorta of Mammalian Embryos.* — 

 V. E. Emmel finds in embryos of mouse, rabbit, and pig certain 

 well-defined cell-masses or clusters in the aorta. Most of the component 

 cells are comparable to the basophilic and phagocytically active cells or 

 macrophags (mesamoeboids ?) in the embryonic circulation. Their 

 constancy of occurrence, firm attachment, and restriction to the ventral 

 wall of tile aorta show that these clusters are not chance accumulations. 

 It is concluded that they have arisen from the vascular endothelium. 



An intimate association and causal relationship can be demonstrated 

 between the formation of the aortic clusters and the developmental 

 processes involving the atrophy of certain aortic rami and the 

 establishment of the permanent intestinal arteries of the adult Mammal. 

 The endothelium in degenerating stems of the aortic rami is stimulated 

 (evidently through certain toxic conditions arising in the retrogressive 

 vessels), to phagocytic and proliferative activities giving rise to infra- 

 arterial cell masses constituting a primary source of the origin of the 

 aortic clusters. Thus under certain abnormal conditions endothelial 

 tissue, ordinarily passive, may in both embryo and adult assume 

 proliferative activities. 



Development of Bird's Lung.f— -W. A. Locy and Olof Larsell have 

 made a very careful study of the development of the lung as a whole, 

 and of the 'intrapulmonary bronchial tree. As Campana pointed out, 

 there is no " bronchial tree " in the adult bird's lung in the sense in 

 which this designation is used for mammals. No bronchial twigs of 

 the adult avian lung terminate l)lindly. On the contrary, there is 

 established a network of intercommunicating passages forming bronchial 

 circuits. The resemblance is closer to the capillary connexion between 

 arteries and veins than to a tree. 



From their earliest formation, the lung pouches are lined by 

 endoderm, and this internal cavity is the basis from which hollow buds 

 arise to form the branches of the bronchial tree. The endodermic tube 

 lies in a layer of mesenchyme that is bordered on the surface towards 

 the pleuro-peritoneal cavity by a well-defined layer of mesothelium. 

 Accordingly, the external boundaries of the lung are formed l:)y a wall 



* Amer. Journ. Anat., xix. (1916) pp. 401-21 (2 pls.K 

 t Amer. Journ. Anat., six. (1916) pp. 447-504 (46 figs.). 



