ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 467 



the bile-duct system, and in the absence of lobulation of the kind 

 generally found in higher Vertebrates. But these peculiarities which 

 distinguish the Selachian liver are not manifested until a comparatively 

 late stage in the development. In earlier stages the characters common 

 to the liver in all Vertebrates, but which are often masked or modified 

 in higher forms, are shown with unusual clearness. It is with them 

 that the memoir has mostly to do. 



Structural Unit of Pig's Pancreas.* — George W. Corner defines 

 the structural unit of an organ as the smallest part of it which is 

 regularly repeated in a similar way throughout, and which contains the 

 elemental constituents of the organ ; that is to say (if it is a gland of 

 external secretion), gland substance, duct, and blood-vessels. The length 

 of a blood capillary defines the size of the unit. Ludwig and his pupils 

 showed that all the ca^JJaries connecting the terminal tips of the arteries 

 and veins are of the same length in any one organ. 



The pancreas of the adult pig is formed by the repetition '20,000 to 

 30,000 times of a structural unit about one millimeter in diameter. The 

 unit is more clearly outlined in the foetus than in the adult. Its size is 

 limited to the area of supply of one arteriole. Pressure of fluid injected 

 into the main duct of the pancreas is equally distributed to all the units. 

 By presumption, the reverse is true, that is, all the units deliver their 

 secretion against equal pressure. 



Corner confirms the statement of Laguesse and others that the early 

 pancreatic ducts are plexiform. The main ducts of the pig's pancreas 

 and its branches arise by dilatation of capillary ducts in the primitive 

 plexus, in a manner similar to the origin of arteries and veins from 

 capillaries. 



Mast Leucocytes of Rabbit.j — A. R. Ringoen finds that the bone- 

 marrow of the rabbit contains true mast myelocytes with basophil 

 granules, in addition to other granular leucocytes. The granules of the 

 mast leucocytes are differentiated gradually out of the basophil cytoplasm 

 of mononuclear cells. As they increase in number, the nucleus changes 

 shape and becomes polymorphous. Fully differentiated mononuclear 

 mast leucocytes are never found in the blood or marrow of the adult 

 rabbit. They show no evidence whatever of degeneration. Their 

 granules are formed by progressive differentiation of the cytoplasm. 

 The haernatogenous mast cells of the rabbit form a distinct and in- 

 dependent line of granulocytes, which is in no way related to the 

 eosinophil or special leucocytes, except through the non-granular parent- 

 cell of the bone-marrow. 



Macrophages of Mammals. J— Herbert M. Evans defines the macro- 

 phages as those mononuclear cells, wherever they may be, lining vascular 

 channels, resident in connective tissues, or entirely free, whose proto- 



* Amer. Journ. Anat., xvi. (1914) pp. 207-36 (19 figs.). 



t Anat. Record, ix. (1915) pp. 233-42. 



t Amer. Journ. Phys., xxxvii. (1915) pp. 243-58. 



