IX MOLLUSOA 319 



METHOD OF EECONSTRUCTING ORGANS FROM SERIES OF SECTIONS 



A word on certain methods is here advisable. The method of 

 handling and mailing sections of minute embryos has been fully 

 described in Chapter II. ; but in the examination of sections, the 

 observer, in looking through a series, mentally synthesises the 

 pictures presented by successive sections, and thus forms a conception 

 of the organ of which they each form part. This, with a little 

 practice, is a comparatively easy matter when one is dealing with a 

 bilaterally symmetrical animal or any animal whose body can be 

 compared to a cylinder ; when, however, one is dealing with an animal 

 which is twisted in a spiral fashion, like Paludina, the mental 

 synthesis is a very difficult matter, and, to assist it, the following 

 method of reconstruction is followed. 



The usual way of applying this method is to draw outline 

 sketches of successive sections at a constant magnification, for example 

 200 diameters. If the sections are 5//, thick, i.e. -^wwo ^^ ^ millimetre, 

 which is a usual thickness in dealing with minute objects, then, such a 

 section, being magnified in all its dimensions to the same extent 

 as the picture that is drawn of it, would be 1 millimetre thick. 

 The drawings are therefore transferred to wax plates 1 millimetre 

 thick, and the outline of the particular organ whose course it is 

 desired to study is boldly drawn in a conspicuous colour. Eound 

 this outHne the wax plate is cut away ; and the pieces of successive 

 plates are piled on one another in the proper order, and in this way 

 the solid form of the organ is reconstructed. In theory the pieces 

 from successive plates should fit exactly, but in practice it is found 

 necessary to adapt them to each other by melting the edges with a 

 hot scalpel. 



Prof. Graham Kerr has, however, elaborated a method by means 

 of which practically as good results are obtained with infinitely less 

 labour. Instead of wax plates he used square plates of fine ground 

 glass of appropriate thickness. On these the outlines of successive 

 sections of the organs which it is desired to reconstruct are drawn 

 with a pencil of coloured chalk. The plates are now piled upon each 

 other in proper order in a square glass vessel which is filled with 

 cedar oil of the same refractive index as that of the glass used — 

 both oil and plates being, of course, specially made for the purpose. 

 The result of this arrangement is that the glass becomes absolutely 

 invisible when immersed in the cedar oil, the coloured outlines of 

 successive sections stand out boldly, and the solid form of the organ 

 is conspicuous at the first glance. One great adva,ntage of this 

 method is that the materials, glass plates and cedar oil, can be used 

 over and over again, since the pencil outlines are easily washed off. 



OTHER GASTROPODA 



We shall now examine how far the developmental history which 

 we have described in Patella and Paludina is exemplified in the case 



