358 The Development of the Connective Tissues 



first the elastic fibrils form a network throughout the exoplasm but 

 they gradually shift to its outer border, leaving the prefibrous tissue 

 within. At this time the nuclei and endoplasm lie upon the exoplasm. 

 A further development liberates the nuclei and endoplasm more and 

 more and the elastic fibers come to form a network which encircles 

 bundles of white fibers, to form the characteristic and fully developed 

 connective tissue. 



The youngest elastic fibrils which are stained by Weigert's method 

 form a delicate network of homogeneous fibrils less than one n thick 

 (smaller than the chromatin granule of the nucleus), and at no time 

 are the fibrils composed of a row of granules as described by Eanvier. 

 Eanvier's description of elastic granules in arytenoid cartilage is cor- 

 rect so far as it goes but does not apply to the development of elastic 

 fibers. 



I have studied carefully the development of elastic tissue in an 

 embryo 7 cm. long, which had been hardened in alcohol, thus permitting 

 tests with various digestion ferments. This was not possible with most of 

 the sections I studied, for they were from embryos hardened in Zenker's 

 fluid. Thin sections of the aorta show, when stained by Mallory's 

 method, a beautiful syncytium composed of fibrillated exoplasm M^ithin 

 which there is a network of sharply defined fibrils which stain intensely 

 blue. The elastic membrane of the intima is also stained intensely 

 blue. The arrangement of the network which stains more intensely 

 by Mallory's method is identical with that stained by Weigert's method. 

 If, now, a section is first digested in pancreatin for 24 or 48 hours, the 

 network of the syncytium and the membrane of the intima are no 

 longer present; as is shown in sections which have been stained with 

 either Mallory's or Weigert's method. A shining mass of anastomos- 

 ing fibrils of the exoplasm alone remains intact, nuclei, endoplasm, and 

 elastic fibrils having been removed by the action of the pancreatin. 

 From time to time specimens may be obtained by the action of pepsin 

 in which only some elastic fibers and fragments of nuclei are left. In 

 general, it is as difficult to isolate elastic fibers by the action of pepsin 

 in the tissues of the embryo as it is to isolate them in the adult. 



In thin sections of the embryo 7 cm. long which have been stained 

 successfully with Weigert's elastic tissue stain, Delafield's hrematoxylin 

 and Congo red the relation of white fibrous and yellow elastic tissue to 

 the syncytium is beautifully shown in the adventitia. The two kinds 

 of fibrils alternate in bundles with nuclei and endoplasm lying upon 

 them. The individuaf elastic fibrils may appear as rows of granules, 

 but the granules never leave the field of the microscope while focusing. 



