152 CONNECTIVE TISSUE. 



under the control of vasomotor nerves. The myxomatous tissue 

 is often found to contain spaces, greatly varying in size and num- 

 ber, filled with liquid ; these, doubtless, are secondary formations, 

 so-called cysts. The outer surface of the umbilical cord is 

 covered byi^a single layer of flat epithelia. 



Now, if we compare a transverse section of the umbilical cord with an 

 amoeba (see page 21), the similarity between the two becomes evident. 

 The epithelial coat of the cord corresponds to the continuous layer of living 

 matter in amoeba ; the complex reticulum of plastids corresponds to the 

 simple reticulum of living matter in the amoeba ; the closed cavities of the 

 blood-vessels holding isolated plastids, the blood-corpuscles of the cord, cor- 

 respond to the closed spaces, the vacuoles containing isolated granules of 

 living matter. In fact, the simple amoeba is the representative of the com- 

 plex structure of the umbilical cord, as well as of all other tissues of the 

 human body. 



FIG. 52. UMBILICAL CORD OF A HUMAN FCETUS, NINE MONTHS OLD. 

 CHROMIC ACID SPECIMEN. 



P, bioplasson cords with nuclei at their points of intersection ; JB, partly homogeneous, 

 partly fibrous, basis-substance. Magnified 500 diameters. 



With lower powers of the microscope we recognize in sec- 

 tions of the umbilical cord of a fully developed human foetus, both 

 fresh and preserved and hardened in a chromic acid solution, a 

 relatively coarse reticulum of plastids, Virchow's branching cells. 

 The best sections are obtained from the portion about midway 

 between the vessels and the surface, because nearer the vessels and 

 the surface the reticulum, being very dense, does not admit of dis- 

 tinct demonstration. We see ramifying so-called protoplasmic 

 strings of a delicate granular structure, containing oblong nuclei 



