338 The Development of the Connective Tissues 



side of the heart and lung. In these regions it is formed of an extensive 

 network of exoplasm with nuclei at the nodal points. In other words, 

 there are multipolar cells with anastomoses of their prolongations. At 

 this time the nuclei are oval in shape, without the surrounding endo- 

 plasm as in the tadpole. At certain points there are indications of a 

 beginning of a differentiation into cartilage and into white fibrous 

 tissue, but beyond this there is the simple syncytium. 



All the above may be seen in ordinary thin sections stained with acid 

 fuchsin, but it is better shown in specimens stained with haematoxylin 

 and Congo red. My best specimens Avere obtained by staining the 

 sections with Mallory's connective-tissue stain,* which tinges the nuclei 

 and surrounding endoplasm, if present, slightly red and the exoplasm 

 of the syncytium intensely blue. We have modified this stain some- 

 what by omitting the water and intensifying the bhie. The method 

 now employed in our laboratory, for which we are indebted to Dr. Sabin, 

 is as follows : 



Specimens hardened in Zenker's fluid are cut in paraffin and fixed 

 on the glass slide by the water method. They are then stained with 

 fuchsin, yV psr cent, until they take up a proper amount of color and 

 then without washing are fixed for a few minutes in a saturated aqueous 

 solution of phosphomolybdic acid diluted ten times. They are next 

 washed in alcohol, 95 per cent, and stained a very short time in the 

 following solution: Aniline blue soluble in water, 1; orange G,, 2; 

 oxalic acid, 2; boiling water, 100. Next they are washed in alcohol, 

 95 per cent, blotted, cleared in xylol, and mounted in Canada balsam. 



With this modification there seems to be no difficulty in obtaining 

 an excellent double stain in nearly all cases, which is not so with the 

 ordinary Mallory stain. Washing the sections with water has a ten- 

 dency to remove the red and this is obviated to a great extent by sub- 

 stituting alcohol. The blue in this modification is strengthened in 

 order that the section need not remain in the aqueous blue stain long 

 enough to remove the red. Successful specimens are especially valuable 

 to trace out the exoplasm of the syncytium which is somewhat matted 

 together when stained with hfematoxylin and congo red. 



Digestion of the Syncytium in Pancreatin and Pepsin. 



It is extremely difficult to obtain satisfactory results by digesting 

 sections of embryos in either pancreatin or pepsin. If the test is made 

 with frozen sections the pancreatin causes them to swell into a transpar- 



9 Mallory, Jour, of Exp. Med., Vol. 5, 1901. 



