ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 459 



Cranial Subarachnoid Spaces.* — Lewis H. Weed distinguishes 

 " serous cavities " derived from the coilom and those cavities which 

 have characteristic organs of elaboration and specialized arrangements 

 for the absorption of their individual fluids. Those of the second type 

 are the aqueous chamber of the eye and the cerebro-spinal spaces. He 

 has studied the formation of the subarachnoid spaces in the cranium of 

 the pig. The process involves a dilatation of the early mesenchymal 

 spaces, a disruption and breaking down of certain of the syncytial 

 strands, and a survival of certain selected strands, to form the permanent 

 subarachnoid channels. In addition to this rarefaction of the mesen- 

 chyme, there is a process of condensation in this tissue giving rise 

 ultimately to the arachnoid membrane and reinforcing the trabecular. 

 Associated with the breaking apart of the syncytium of the perimedullary 

 mesenchyme, there are invarialily found large coagula of albuminous 

 material, apparently an index of the circulation through the spaces of 

 the embryonic cerebro-spinal fluid. 



Regeneration in the Tail of Frog Tadpole.f — H. E. Metcalf has 

 made a histological study of the cell changes in the epidermis during 

 the early stages of regeneration in the tail of the tadpole of Rana 

 damitfms. The amphibian tail was chosen because of the size of the 

 ■cells, and the ease wdth which they are studied under the Microscope. 

 The special aim of the paper is to give an account of the migration of 

 the epidermal cells to cover the wound, and to describe the cytoplasmic 

 and nuclear changes in the migrating cells. Within one laour after 

 cutting there is a muscle contraction, which decreases the surface of 

 the wound and brings the epidermis of the two sides of the tail together 

 both above and below the notochord, where the epidermis fuses. There 

 is a migration of the epidermal cells over the blood-clot which is 

 always formed within twelve hours. This is an active migration of 

 the epidermal cells, especially those of the outer layer, with no sign of 

 multiplication either by mitotic or amitotic cell-division. There is a 

 decrease in the ratio of cytoplasm to nucleus in the migrating epidermal 

 cells, as compared with that in the normal epidermis. The ratio seems 

 to increase slightly as the cells begin to leave their regular positions, 

 and then to decrease steadily in the actively migrating cells. As the 

 cells are removed from their normal position in relation to the blood- 

 vessels, this decrease may be due to starvation. The rate of meta- 

 bolism in the individual cells is probably high, and they may be forced 

 to rely on the energy contained in the cytoplasm, and so use it up during 

 migration. If the views of Minot and Conklin can be applied to these 

 cells, the decrease in the ratio of cytoplasm to nucleus would indicate a 

 rejuvenescence of differentiated epidermal cells. Migration ceases at 

 about twenty-four hours, and the ratio returns to normal in the epi- 

 dermal cells. The period of time from twenty-four to forty-eight hours 

 is occupied by the absorption of the clot, and the cells are in a resting 

 condition. At forty-eight hours regeneration by mitosis begins. No 



* Anat. Record, x. (1916) pp. 475-81. 



t Trans. Amer. Micr. Soc, xxxiv. (19] 5) pp. 167-84 (10 figs.). 



