Franklin P. Mall 3i9 



surrounding endoplasm running out into two poles to be lost in the 

 exoplasm. 



Digesting the syncytium and the prefibrous tissue in pancreatin shows 

 that they resist its action to a marked extent. In order to obtain 

 any satisfactory result the digestion must be mild, i. e., for a short time- 

 at room temperature. Great quantities of resistant white fibers cannot 

 be obtained from the skin by means of digestion in pancreatin until the 

 embryo is about 15 cm. long. Although no elastic fibers can be demon- 

 strated in the skin of embryos 15 cm. long, frozen sections of it will re- 

 sist boiling acetic acid for a very long time. In embryos 25 mm. long the 

 first prefibrous tissue of the perimysium resists pancreatin more than 

 the remaining syncytium. The fibrils of the prefibrous tissue also swell 

 in dilute actic acid. These reactions, together with the position of the 

 tissue in question, make it very definite that the changes in the syn- 

 cytium immediately over the muscle mark the beginning of white fibrous 

 tissue. 



The prefibrous tissue of an embryo 2 cm: long is formed of anastomos- 

 ing fibrils which are in direct connection with the exoplasm lying 

 between the perimysium and the ectoderm. Immediately below the 

 ectoderm the meshes of the exoplasm are smaller, the syncytium thus 

 forming a more compact felt upon which the epithelium rests. 



In embryos 3 cm. long the perimysium is a little more compact and 

 sends reflections between the muscle fasciculi. The layer immediately 

 below the ectoderm is a little more extensive than before, while between 

 it and the perimysium the syncytium is quite typical. The two zones 

 of altered exoplasm have approached each other in an embryo 4 cm, 

 long, leaving a narrow zone of typical syncytium between them, within 

 which the first lymph channels have appeared. 



In the next stage, 5 cm. long, all of the exoplasm of the syncytium 

 of the skin has changed considerably, being more fibrillated, with some 

 of the fibrils staining more intensely than the rest (Fig. 13). The 

 prefibrous perimysium is advanced one step more now, being composed 

 of layers of fibrillated exoplasm, the layers anastomosing between them- 

 selves, and the fibrils within a given layer forming a dense network. 

 True white fibrous tissue is not yet present. 



The prefibrous tissue has extended still more in an embryo 7 cm. 

 long. Its development is most advanced in the perimysium, where the 

 individual fibrils are beginning to become wavy. In a transverse section 

 of the body it is seen that the development is most advanced in the 

 neighborhood of the vertebral column and least in the umbilical cord. 

 From without inward the permysium is most developed, diminishing in 



