438 SUMMAP,Y OF CUEKENT RESEARCHES RELATING TO 



canals, accomplished by the genesis of active mesoblastic tissues on and 

 within the lime deposit, the mesoblastic tissue being derived from 

 adjacent blood-vessels of the ovary; and (4) true bone formation with 

 marrow. Blood-vessels, osteoblasts, bone-cells, and marrow (in large 

 part, at least), are merely differentiations of the mesenchymal cell unit. 

 The histological constituents of the new blood-vessels are the progenitors 

 of all the histological components of the osseous tissue. The cases tell 

 against the theory of the specificity of endothelium, and favour the 

 adaptive or mesenchymal theory. Ossification does not occur without 

 preliminary calcification, and calcification occurs only in dead tissues. 

 There is no valid reason for regarding bony structures within the ovary 

 as blastomata. 



Bone Formation in Ovary.*— G. W. Outerbridge describes a number 

 of cases in which true bone developed in the ovary, independently of 

 neoplastic or teratomatous processes, in most if not all cases in corpora 

 fibrosa. 



Dwarf-eggs of Domestic Fowl.t — Raymond Pearl and Maynie R. 

 Curtis have studied the production of dwarf-eggs in the domestic 

 fowl. They find two distinct types of shape, a prolate spheroidal and 

 a cylindrical, the latter being infrequent. Thirty-five per cent of the 

 eggs examined were yolkless ; the rest contained yolk, but only in 

 9 p.c. of these was there an enclosing membrane. The production of a 

 dwarf-egg is usually an isolated phenomenon, occurring only once or 

 twice in the lifetime of a bird, and in most cases the occurrence is not 

 correlated with a morphological disturbance of the sex-organs. In eleven 

 out of two hundred cases, however, no normal eggs were produced after 

 the dwarf-egg, and examination showed, in five of these, some patho- 

 logical condition of the oviduct which interfered with the passage of the 

 egg. In normal birds dwarf -egg production occurs during the height of 

 the breeding season, and is not associated with immaturity. Dwarf -eggs 

 may be produced by stimulation of an active duct by some material 

 which is not yolk, but at least 65 p.c. of those studied were initiated by 

 an abnormal small yolk, or by part of a normal yolk, the rest evidently 

 being absorbed by the visceral peritoneum. 



The Term "Factor."^ — Howard B. Frost points out that the term 

 "factor-" has, in genetic use, two distinct meanings, which are continually 

 interchanged or combined, and often confused. It is essential to 

 clearness in genetic discussion that these two meanings should be care- 

 fully distinguished. The following formal definitions are suggested : 

 1. A genetic (Mendelian) factor is a property or characteristic of the 

 germ-plasm, more or less conveniently delimited for the purpose of 

 analysis of segregating heredity. 2. A genetic (Mendelian) factor, or 



* Amer. Journ. Med. Sci., cli. (1916) pp. 868-87. See Physiol. Abstracts, i. 

 (1916) No. 7, p. 286. 



t Rep. Maine Agric. Exper. Stat., 1916, pp. 289-328. 

 J Amer. Nat., li. (1917) pp. 244-50. 



