244 SUMMARY OF CURRENT RESEARCHES RELATING TO 



in their osmotic pressure. The low osmotic pressure in the blood of 

 Amphibians and some higher Vertebrates points to a derivation from 

 fresh-water ancestors. In Mammals' blood there are more salts than in 

 Amphibians' blood, but the proportions of salts of sodium, potassium, 

 calcium and magnesium is precisely parallel to the proportions in sea- 

 water, as Quinton has shown. This is a record of the ancestral marine 

 habitat. J. A. T. 



Evolution of the Chin. — T. T. Waterman {Amer. Naturalist, 1916, 

 237-42), Walkhoff and Robinson have interpreted the chin as a result 

 of articulate speech, for it distinguishes modern man from Primates and 

 even from the Heidelberg man. But the elephant seems to have a 

 very pronounced chin, if the mental process is really homologous with 

 man's. But Waterman's idea is that the human chin is a residue of a 

 much larger mandible in ancestral forms. The reduction has been 

 unequal, more on the upper margin (associated with smaller and more 

 closely packed teeth) than on the lower margin of the mandible. The 

 chin is a relic, not a new acquisition. J. A. T. 



Dentigerous Cyst on Cod's Upper Jaw. — H. Chas. Williamson 

 {Journ. Pathology and Bacteriologrj, 1919, 22, 255-6, 2 pis.). A 

 condition of rare occurrence is described. A hard spherical tumour 

 projecting from the right maxilla was found to consist of a bony capsule 

 covered with a thick, fairly soft, rugose skin, and contained between the 

 periosteum and the dermis a dark-red layer of tissue bearing teeth, while 

 inside the cyst there were no fewer than seventeen hundred. The 

 tumour had apparently arisen through the persistence on the anterior 

 side of the premaxilla, between the periosteum and the bone, of a 

 portion of dentigerous tissue. J. A. T. 



Fossil Fish-scales.— T. D. A. Cockerell {Amer. Naturalist, 1917, 

 51, 61-3). Attention is directed to the work of Geinitz on fish-scales 

 and to his careful drawings. " Although Geinitz knew little about the 

 affinities of his scales, they had excellent characters, reminding us in 

 certain cases of modern genera, and indicating the great antiquity and 

 constancy of peculiarities of scale structure." The author pleads for a 

 more systematic and critical study of fossil lish-scales. J. A. T. 



Tunicata. * 



Nervous System of Ascidian.— Edward C. Day {Journ. Exper. 

 Zool., 1919, 28, 307-35, 5 figs.). The margins of the siphons of 

 Ascidia mentula are the most sensitive part of the animal, closing when 

 stimulated. Feeble stimulation may affect a single lobe, stronger 

 stimulation the whole siphon, stronger still the other siphon as well. In 

 a siphon partially slit longitudinally each half responds locally to a 

 feeble stimulus, while stronger stimulation sends the impulse around the 

 cut, and produces a response of the two halves in sequence. Amputated 

 siphons retain their sensitiveness for five or six days and then die. 

 They are replaced, and the ganglion is not necessary for the regenera- 



