170 SUMMARY OF CURRENT RESEARCHES RELATING TO 



and in the yolk-sac are devoid of phagocytic function ; and they do not 

 undergo mitosis, either complete cytoplasmic or incomplete nuclear. 

 Once the hgemogenic giant-cells of the red bone-marrow have passed 

 beyond the stage with features characteristic of the hfemoblast, their 

 further history only leads through progressive steps toward disintegra- 

 tion, involving terminally in some cases nuclear appearances simulating 

 multipolar mitotic figures. J. A. T. 



Dental Star in Horses and Cattle. — E. Retterer {Conijjtes 

 Rendus Soc. Biol., 1020, 83, 1069-72). The teeth of horses and 

 cattle continue to form ivory, even after the layer of odontoblasts 

 has disappeared. The ordinary, rounded or stellate, cells of the papilla 

 preside at this formation of ivory. The dentine thus formed exhibits 

 some of the characters which pathologists have described under the 

 name of secondary dentine. It is traversed by calcified globular 

 masses, interrupting the course of the canaliculi. No cells were visible. 

 In man's teeth the secondary dentine is formed by large osteoblasts ; in 

 horse and ox the ordinary tissue of the papilla forms the secondary 

 dentine. Mechanical factors influence its development ; where there is 

 strong pressure it turns into enamel ; where it is sheltered it turns 

 yellow or black and begins to show disintegration. The dental star is 

 ivory at the beginning of caries. J. A. T. 



Retina of Alligator. — H. Laurens and S. R. Dbtwiller {Jomn. 

 Exfer. Zool, 1921, 32, 207-34, 13 figs.). The eye of Alligator 

 mississippiensis shows a well-developed retinal tapetum in the dorsal 

 and posterior portions of the retina to within 1 • 5 mm. of the entrance 

 of the optic nerve. It is due to the inclusion of guanin in the cells of 

 the epithelial layer. The pecten is a slightly raised pigmented cap 

 covering the entrance of the optic nerve. Typical rods and cones 

 occur, the ratio differing in different regions. The rods are all of one 

 type. There are thick and thin cones, none with oil drops. The 

 authors discuss the change in the length of the rods and cones, and the 

 change in the position of the pigment between light and darkness. A 

 theoretical consideration of photomechanical changes and of the duplicity 

 theory from the comparative point of view is appended. J. A. T. 



Oviducal Glands of Chelonia. — R. Argaud {Comptes Rendus Soc. 

 Biol., 1920, 83, 828-9). There is a double equipment : («) mucus- 

 making unicellular glands due to a temporary transformation of ciliated 

 cells, and {h) definitive glands in the form of ramified tubes. The 

 very granular cells of the tubular glands are differentiated at sexual 

 maturity, and probably serve to secrete protective membranes for the 

 ovum. J. A. T. 



Grunhagen's Spaces. — M. A. Herzog {Revue Suisse Zool., 1920, 

 82, 99-113). The spaces below the villous epithelium of the intestine 

 have been studied experimentally and histologically, with the result that ' 

 the investigator is quite convinced as to the artefact nature of Griinhagen's 

 spaces. J. A. T. 



