362 The DeveloiDment of the Connective Tissues 



filled with nuclei encircled with endoplasni lying upon a delicate net- 

 work of exoplasm. This is beautifully showai in specimens stained by 

 Mallory's method, and also to a certain extent by Weigert's method, 

 provided the stain is pushed until the surrounding white fibrous tissue 

 stains also. In embryos a little over 25 cm. long the elastic tissue of 

 the arteries of the skin has increased in quantity, and the exoplasm of 

 the syncytium below the roots of the hairs undoubtedly is stained more 

 readily by Weigert's method than before. 



When frozen sections of the skin (which show no elastic tissue by 

 Weigert's method) are boiled in dilute acetic acid until the white 

 fibrous tissue is either dissolved or converted into a jelly mass, a net- 

 work of sharp fibers can still be demonstrated. by staining the swollen 

 section with magenta or with very dilute gentian violet (Fig. 15). In 

 case the sections are not boiled very long the gelatinous exoplasm of 

 the syncytium has imbedded within it sharp fibrils upon which lie 

 oval nuclei surrounded with a plate of endoplasm. When the boiling 

 is pushed still further, until the section falls nearly into pieces, it can 

 still be coaxed upon the glass slide and stained with magenta under the 

 coverglass. The main bands of syncytium are now practically all dis- 

 solved, leaving a network of delicate and sharply defined fibrils which 

 appear to be directly continuous with the endoplasm around the nuclei. 

 Often some of the anastomosing fibrils are quite free and upon them 

 lie the nuclei and surrounding endoplasm (Fig. 18). These specimens, 

 which are extremely instructive, show definitely that the nuclei and 

 endoplasm lie upon the fibers. Furthermore, when frozen sections are 

 treated a short time in boiling dilute caustic potash only a network — 

 the elastic fibers — remains, all the rest, including the nuclei, having 

 been dissolved. These tests show that an elastic network is present 

 in the skin in young embryos before it can be stained by Weigert's 

 method. Elastic tissue can be demonstrated by Weigert's method in 

 the skin of the embryo pig about 25 cm. long, and by maceration in 

 boiling acetic acid, and staining with magenta, the fibrils can easily be 

 isolated (Fig. 16). It is therefore seen that elastic tissue is present in 

 the skin long before it can be stained by Weigert's method. 



In a section of the skin in Avhich the elastic fibers just begin to take 

 the Weigert's stain it is seen that the bundles of white fibrous tissue 

 are accompanied by one or two elastic fibers. In the region of the 

 roots of the hairs, where the development is not so far advanced, the 

 fibers are continued into the exoplasm of the syncytium and are related 

 to the nuclei and endoplasm as described above. Frozen sections boiled 

 in acetic acid (1 per cent) until very soft, then coaxed upon the glass 



