IMBEDDING METHODS. 85 



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

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

 But some objects, such as ova of Crustaceaj may require 

 three or four days. (Heidecke, Juna. Zeit., xxxviii, 1904, 

 p. 506; Mayer, Grundzuge, Lee and Mayee, 1910, p. 85; 

 Beinkmann, Mitth. Zuol. Btat. Neapel, xvi, 1903, p. 367, 

 three to five days for uterus of Selachians : Mullee, Arch, 

 mikr. Anat., Ixix, 1906, p. 3, for lungs of mammals ; Peso, 

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

 days 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 ivarm melted paraffin 

 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-bath 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 

 Jourti. Boy. Mic. Soc, 1883, or Oaepbntbb's The Microscope, p. 452. 

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

 described in Gentralbl. Bdkt., xlv, 1907, p. 191 (see Journ. Soy. Mic. Soc, 

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

 especially Jxtngj, and Gktjblek and Hollboen; and the descriptions in 

 the technical journals. 



141. Imbedding in Vacuo.— 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 as 

 may easily happen with objects possessing internal cavities if it be 

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

 of any ai-rangement that will allow of keeping paraffin melted under a 

 vacuum. 



That of Hoffmann is described and figured at p. 230 of Zool. Ana., 

 1884. In this arrangement the vacuum is pi'oduced 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 with 

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



