66 MONTROSE T. BURROWS 



METHODS 



Dissection of the tissues 



The methods of dissection of the nervous system, muscle plates 

 and other tissues were similar to those used by Harrison in the 

 frog embryos. The preservation of the specimens from bacterial 

 infection has been a comparatively simple matter. The observ- 

 ance of simple surgical technique and the use of freshly sterilized 

 glass-ware and instruments for each operation are sufficient. To 

 prevent long periods of chilling, the solutions and dissecting watch 

 glasses were kept at 39° C. The dissections were made under a 

 binocular microscope, which was completely covered by a warm 

 oven, heated to 39° C. 



The course of an operation for the preparation of specimens 

 from an embryo may be summed up as follows : The egg is taken 

 from the incubator, and the embryo in its membranes quickly 

 removed with sterile instruments to warm Ringer's solution under 

 the binocular microscope. The membranes are removed. The 

 heart, now quite free from the surrounding tissues, is next removed 

 with scissors by cutting through the base of the vitelline vein 

 and the truncus arteriosus. The dissection of the nervous system 

 is now begun. The specimen is balanced with forceps ; two lateral 

 incisions of the skin are made with scissors just ventral to the 

 nervous system; the optic cups are excised close to their connec- 

 tions with the neural tube ; the skin and the mesenchyme are care- 

 fully removed with needles; the cord is now cut in the upper 

 cervical region and this piece of neural tube, comprising brain 

 and upper cervical cord, is floated off free into the solution. The 

 tube shows a round normal shape, open at the points of attachment 

 of the optic stalks, at the end of the cut cord and at the roof of 

 the fourth ventricle, which has been removed with the skin dur- 

 ing the dissection. Brains were transferred to plasma in this 

 condition in all the earlj/ experiments. In later preparations a 

 hemi-section of the nervous system was made with sharp scissors 

 along the median sagittal plane. The reason for this procedure 

 will be discussed later. 



