34 INVERTEBRATA CHAP. 



very good stains. The transference to oil of cloves must be made by 

 adding this substance drop by drop to the absolute alcohol at intervals 

 of an hour or so for several days. After a sojourn in the pure oil 

 the specimen is placed in the concavity of a hollowed slide and 

 suddenly covered with thick solution of Canada balsam in xylol. 

 The oil of cloves flies to the periphery of the balsam owing to 

 surface tension and may be removed by blotting-paper. 



When all the information possible has been gleaned from whole 

 mounts of embryos and larvae the next step is to cut them into 

 series of sections arranged in order, but for this purpose they must lie 

 embedded in a block of paraffin so that the sections when cut by the 

 microtome will lie parallel to a known direction. To accomplish this 

 placing, or orientation, as it is called, in the case of minute larvae is 

 a matter of great difficulty, and unless the sections are cut in the 

 right direction they are very difficult to interpret. The best way to 

 overcome this difficulty is to embed the specimens in celloidin 

 before embedding in paraffin. The solution of celloidin used for 

 embedding vertebrate tissues, consisting of celloidin dissolved in 

 a mixture of equal parts of absolute alcohol and ether, is not 

 suitable for delicate larvae because too violent diffusion currents 

 are produced in the process of changing from alcohol to the 

 celloidin solution. If, however, the celloidin be dissolved in a 

 mixture of four parts of absolute alcohol and one part of ether, 

 then such currents are avoided. It is well to have three grades of 

 this solution, one saturated, one made by diluting the saturated 

 solution with an equal bulk of the solvent, and one by diluting it 

 with two volumes of the solvent. The objects, if they are small, 

 should remain in each grade for about one day. Then the thick 

 solution with its contained embryos is poured into chloroform and 

 the celloidin hardens to a cheesy consistence. After an hour's 

 sojourn in this fluid a piece of celloidin containing the embryo 

 can be cut out and embedded in paraffin. 



The embedding may be done in one of two ways. 



(1) The piece of celloidin containing the object is placed in 

 absolute alcohol, to remove any trace of moisture, and then 

 immediately transferred to fresh chloroform to which fragments of 

 clean paraffin are added. If the whole be heated to 60 for an 

 hour all the chloroform will have evaporated and the object can now 

 be poured, together with some of the paraffin, into a mould and 

 allowed to cool. Before transferring to the mixture of chloroform 

 and paraffin, the object can be studied under the lower power of the 

 microscope and the celloidin shaped so as to direct the orientation of 

 the block of paraffin. 



(2) The object and its surrounding celloidin may be transferred 

 to cedar oil. If this lie warmed for half an hour (by being placed on 

 the top of the thermostat) all traces of moisture will be absorbed, 

 and the cedar oil will render the celloidin absolutely transparent, 

 so that the object can be examined as if it were mounted in oil of 



