CHAPTER VIII. 

 METHODS. 



Measuring Length of Embryos. 



Owing to the many changes during development in the curvature of the 

 longitudinal axis of the mammalian embryo, it is impracticable to measure that 

 axis or to employ any one system of measurements to obtain comparable results 

 for all ages. For this reason the best practice is to measure in all cases the 

 greatest length of the embryo in its natural attitude along a straight line. The 

 limbs are not to be included in such measurements. This greatest length in 

 young stages will not include the head (compare, for example, Fig. 80), but in 

 most stages the head would be included. Many German authors employ the mea- 

 surements introduced by His, which he calls the Nackenlange, which corresponds 

 to the distance in a straight line from the neck-bend to the caudal bend. As it is 

 impossible to measure this distance in later stages, it seems best not to use it at 

 all. The length of an embryo, as given by German authors, is often indicated by 

 the abbreviation NL., and is, of course, often different from the measures used in 

 this work. 



Methods of Reconstruction. 



It is often important to obtain definite plastic conceptions of the anatomy 

 of embryos or parts of embryos too small for dissection. To secure these in the 

 best form, it is necessary to reconstruct either drawings or models from sections. 

 The methods employed for these two forms of reconstruction, being difi'erent, 

 must be described separately. 



Reconstruction of Drawings from Sections. — To make these reconstructions 

 satisfactory, it is indispensable to have an accurate outline of the embryo repre- 

 senting it in the point of view to be used for the reconstruction and enlarged to 

 the precise scale upon which the reconstruction is to be made. This drawing 

 must, of course, be made before the embryo is imbedded and sectioned. It is 

 further necessary to know accurately the plane of the sections and their thick- 

 ness, and, finally, the total number of sections in the series must be counted. A 



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