348 The Development of the Connective Tissues 



ment of the connective tissues in the skin on the dorsal side of the 

 body, between the two shoulder blades. Here there is the underlying 

 broad trapezius, which marks a region in the section of the skin on one 

 side with the epidermis on the other. Some 50 stages of this region 

 were cut from embryos measuring from 1 to 30 cm. AIJ: important 

 stages were hardened in Zenker's fluid, stained by Mallory's method, as 

 well as by other methods. A parallel set of specimens was also made by 

 macerating them in Midler's fluid for 24 hours, washing, hardening, etc., 

 then staining in hsematoxylin and congo red. All the stages were also 

 frozen, cut and examined fresh, then treated with dilute acetic acid and 

 with caustic potash, after which they were stained with magenta and 

 other aniline dyes. While the specimens stained by Mallory's method 

 give the most definite and permanent preparations, the macerated and 

 fresh preparations give an excellent control. Mallory's method stains 

 pretty much all connective tissues in the embryos, while with macera- 

 tions and digestions there is some differentiation. 



It is shown by means of digestion in pancreatin that the earliest 

 definite fibrils in the syncytial exoplasm are resistant in fresh specimens 

 as well as in hardened specimens which have been fixed upon the glass 

 slide. In acetic acid and in caustic potash the exoplasm and its colla- 

 genous derivatives become transparent in embryos 25 mm. long. In 

 larger embryos there is a residual syncytium which resists acetic acid, 

 even when boiled in it for hours, and probably is related in some way 

 to yellow elastic tissue. This will be considered later. 



In general the connective-tissue syncytium is fully developed in 

 embryos 15 mm. long. It is practically of equal density throughout the 

 skin. The nuclei are mostly round or somewhat oval, usually quite 

 naked or with only a small amount of endoplasm around them. The 

 exoplasm is very delicate, with a slight amount of fibrillation. 



In an embryo 2 cm. long the connective-tissue syncytium has begun 

 to differentiate; many nuclei are oval in shape and they are enveloped 

 with an increased quantity of endoplasm, which runs out on either side 

 of the nucleus, forming two poles— the well-known connective " tissue 

 cells " of the embryo. The nuclei lying just below the ectoderm have 

 the least quantity of endoplasm around them. As the muscle lying 

 under the skin is approached the nuclei increase in number, forming 

 quite a dense layer over it. Analyzing this layer by means of Mallory's 

 stain shows that it is formed of syncytium which is drawn out parallel 

 with the long axis of the body. The fibrillated exoplasm tends to 

 form parallel bundles which anastomose quite frequently with one an- 

 other. In this region the nuclei are markedly spindle-shaped, Avith the 



