CHAPTER VIII. 



METHODS. 



Measuring Length of Embryos. 



Owing to the many changes during development in the curvature of the. longi- 

 tudinal 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. 95), but in most stages the 

 head would be included. Many German authors employ the measurement intro- 

 duced by His, which he calls the Nackenldnge, 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. 



Dissection of Embryos. 



Vertebrate embryos may be dissected without serious difficulty. Specimens 

 hardened in Zenker's fluid are particularly favorable for this purpose. Dissection 

 should be more extensively practiced than is at present usual in embryological work, 

 since by it all the viscera, the central nervous system and even the main nerve 

 trunks may be demonstrated advantageously. 



By the following simple method the embryo may be securely attached to 

 the bottom of the dish in which the dissection is to be made, best in 80 per cent 

 alcohol. Place the specimen in 95 per cent alcohol for half an hour, then in a 

 mixture of equal parts of alcohol and ether for fifteen minutes. Put a few drops 

 of celloidin dissolved in alcohol and ether on the bottom of the dish; put the 

 specimen in place; allow the celloidin to stand for two or three minutes until a 

 film is formed and then cover the specimen with 80 per cent alcohol, which will 

 set the celloidin. After the dissection is finished the specimen may be freed by 

 dissolving the celloidin with a mixture of alcohol and ether in equal parts. 



Methods of Hardening and Preserving. 



The three most generally useful methods for preserving embryos are with Zen- 

 ker's or Tellyesnicky's fluids and 10 per cent formalin. Good results may be had 



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