and Laboratory Methods. 



2671 



Fig. a. — Pieces of 

 Ulva with objects 

 oriented on tlieni. 

 The guide -edges 

 are here represent- 

 ed by thicker lines. 



is placed on a ruled slide {t'ide infra) under a dissecting microscope, A very 

 little celloidin-clove-oil is smeared on the alga and the object, clarified with clove 

 oiP, is laid on it. Care should be taken to transfer as little oil as possible with 

 the object. Sometimes Drew's method of draining off the oil is found ^ 



very useful, but for this purpose tracing linen is not necessary, a glass 

 plate or a piece of glazed paper answering just as well. Now as to 

 orientation. Two cases may be distinguished ; there is a certain 

 kind of object in which orientation in only two dimensions or axes 

 is necessary. To this belong rather flat or elongated objects like 

 a Nereis larva of three segments. There are, on the other hand, other 

 objects in which orientation in three axes is required ; these often have 

 these axes of nearly equal length, as in a Polygordius larva with but slightly 

 elongated post-trochal region. In the former case all that is necessary is to 

 move the object to make one of its axes parallel to one edge (the guide-edge) 

 of the Ulva piece. Then a drop of xylol or cedar oil is poured on the object, 

 which fastens it firmly to the Ulva. The slide is placed under a compound 

 microscope and examined whether the object is correctly oriented. If not, the 

 Ulva is cut in definite relation to an axis of the object. In the second case, 

 when orientation in three dimensions is necessary, the object is first moved by 

 means of a needle until one of its axes comes into a plane perpendicular to the 

 Ulva surface. The subsequent processes are the same as in the first case. But 

 quite often objects of this kind cannot be kept long enough in a desired position. 

 In such cases it is better to drop xylol or cedar oil on the object as soon as one 

 of its axes comes in a vertical plane, and subsequently to cut the Ulva so as to 

 make one side (the guide-edge) parallel to the desired plane of section of the 

 object. As before, the piece should be examined with a higher power, and the 

 orientation controlled. The two corners opposite to the guide-edge (Fig. A. a. and 

 b.) are then cut off in order to avoid confusion between the guide-edge and the 



others during the following manipulations. (In the 



figure the guide-edges are represented by thicker 

 lines.) 



Here I shall describe a little device for cutting 

 the edge of Ulva exactly parallel or perpendicular 

 to one of the axes of the object which has been 

 fastened on the alga. The Ulva piece is placed on 

 a ruled slide 2, /. <?., a slide having two sets of par- 

 allel lines, say '1 mm. apart, intersecting one another 

 at right angles, as is shown in Fig. B. The Ulva is first moved so as to make 

 an axis of the object fall on one of the parallel lines of the slide, and then cut 



^ Since prolonged stay in clove oil is rather injurious to the objects, it is always better to 

 use cedar oil first for clarification, and just before orientation to place the object in clove oil for 

 a few minutes. In this way shrinkage is prevented to a great extent. Cedar oil should be used, 

 especially when the object is to be studied in toto before sectioning, since in this the object can 

 be kept indefinitely without danger of becoming brittle (Cf. Bolles Lee. A. Zool. Anz., 

 8 Jg. p. 563, 1885, and Vade Mecum 5th ed., pp. 81, 97 ; Hoffmann, R. W., Zeit. wiss. Mikr. Bd. 

 17, p. 446, 1 90 1). 



-The ruled slide can be had from the Bausch & Lomb Optical Co. This slide may be used 

 for other purposes, e.g., measuring small objects when used with an ocular micrometer, count- 

 ing the number of small organisms and arranging objects in regular rows for total preparation. 



Fig. B. — Piece of Ulva with object 

 on it placed on a ruled slide. 



