CARE AND UTILIZATION OF THE COLLECTION. 31 



can not be treated in this way. Such a specimen is cut into sections and from the 

 sections an effort is made to reconstruct the anatomy. In the first publication 

 the anatomy of the embryo was worked out by Bern's (1883) method of recon- 

 struction, but unfortunately at that time this method gave little more than the 

 external form, the finer structures being lost in the model. The plan of dissecting 

 the model was then tried, and it soon became clear that success in reconstruction 

 depended very largely upon one's power to visualize the structures from the serial 

 sections, and to some extent upon inventive ability in eliminating a part of the 

 model, as deeper structures can not be shown without removing the superficial 

 ones. Gradually we evolved what we term dissectible wax models, which, however, 

 are somewhat clumsy and usually warp in warm water. Finally, Dr. Bardeen 

 (1901), who is skilled in work of this kind, discovered that excellent results could 

 be obtained by making a foundation model in wax and then projecting and elaborat- 

 ing it as a drawing by the graphic method of His (1892), thus combining the good 

 features of two very valuable methods. 



At first our record of sections was made by drawing them with the camera 

 lucida or, better still, in a dark room by the aid of a magic lantern. This, however, 

 is a very laborious and time-consuming task, and has a tendency to check the 

 mental activity of the investigator, for unfortunately the work can not be done 

 successfully by a technical assistant. When the collection was transferred to the 

 Carnegie Institution we availed ourselves of the opportunity to install an accurate 

 projection apparatus patterned somewhat after the one used by His (1892). With 

 this apparatus sections of embryo No. 460, previously referred to, were photo- 

 graphed on glass negatives with an enlargement of 50 diameters. Thus it was 

 possible to make several prints. As the photographs were taken with a 50-mm. 

 planar lens, they show all the details wonderfully well. 



As everything which we wished to reconstruct had to be transferred to wax, it 

 was highly desirable that the final model should be composed of some temperature- 

 resisting substance. A method has therefore been devised whereby all the struc- 

 tures desired are eliminated from the wax. Thus, in a block of wax the model is 

 represented in outline by a hole or a series of holes, according to the number of 

 structures to be reproduced, and these holes are subsequently filled with plaster 

 of pans. When this hardens and the wax is removed, the finished plaster model 

 is liberated. 



This process of modeling depends solely upon absolute accuracy in super- 

 posing the wax plates to correspond to the original sections of the embryo. Hence 

 a perpendicular line must be established, and this is done as follows: The wax 

 plates of the sections are placed one upon another until a complete model of the 

 embryo is built up (fig. 3), the construction being guided by photographs of the 

 embryo made before it was cut. Thus it is possible to duplicate accurately the 

 external form of an embryo on an enlarged scale. After this is done it is a simple 

 matter to mark the wax plates in a perpendicular direction that is, by right- 

 angle lines drawn upon every plate through its axis. These constitute what we 

 call guide-lines, the instrument for marking which was invented by Dr. W. H. 



