IMBEDDING METHODS. 85 



after an hour's bath, or often less. Many workers habitually 

 give much longer baths, I think often longer than necessary. 

 But some objects, such as ova of Crustacea, may require 

 three or four days. (HEIDECKE, Jena. Zeit., xxxviii, 1904, 

 p. 506; MAYER, Grundziiye, LEE and MAYER, 1910, p. 80; 

 BRINKMANN, Mitth. Zool. Stat. Neupel, xvi, 1903, p. 367, 

 three to five days for uterus of Selachians : MULLER, Arch. 

 miJcr. Anat.j Ixix, 1906, p. 3, for lungs of mammals; Poso, 

 Esperieuze micro techniche, Napoli, 1910, p. 29, five to twelve 

 clays for uterus and placenta of Homo.) I take as a guide, 

 generally, the length of time the object has taken to clear 

 in the cedar oil, assuming that the warm melted parallin 

 ought to penetrate at least as quickly as the cold oil ; 

 and then allowing somewhat longer, say as much again, 

 in order to be on the right side. 



140. Water-baths and Ovens. It is important that the paraffin 

 should not be exposed to a moist atmosphere whilst it is in the liquid 

 state. If a water-hath be used for keeping it at the required tempera- 

 ture provision should be made for protecting the paraffin from the steam 

 of the heated water. 



A very convenient apparatus for this purpose is that of Paul Mayer, 

 or " Naples water-bath," which will be found described at p. 146 of 

 Jour n. Roy. Mic. Soc., 1883, or CARPENTER'S The Microscope, p. 452. 

 An extremely simple stove, which anyone can make for himself, is 

 described in Gentralbl. Bcikt., xlv, 1907, p. 191 (see Joum. lloij. Mic. Soc., 

 1908, p. 109). For others, see the price-lists of the instrument makers, 

 especially JUNG, and GKUBLER and HOLLBORN; and the descriptions in 

 the technical journals. 



141. Imbedding IN YACUO. There are objects which, on account of 

 their consistency or their size, cannot be penetrated by paraffin in the 

 ordinary way, even after hours or days in the bath. For such objects 

 the method of imbedding under a vacuum (strictly, under diminished 

 atmospheric pressure) renders the greatest service. It not only ensures 

 complete penetration in a very short time a few minutes but it has 

 the further advantage of preventing any falling in of the tissues, such aa 

 may easily happen with objects possessing internal cavities if it 

 attempted to imbed them in the ordinary way. It is realised by means 

 of any arrangement that will allow of keeping paraffin melted under a 

 vacuum. 



That of HOFFMANN is described and figured at p. 230 of Zool Anz., 

 1884. In this arrangement the vacuum is produced by means of a 

 pneumatic water aspiration pump, the vessel containing the paraffin 

 being placed in a desiccator heated by a water bath and furnished \vitli 

 a tube that brings it into communication with the suction apparatus. 



