22 HISTOLOGY. 



it is vestigial in the adult. It sometimes develops abnormally, forming a 

 peculiar tumor. 



The mesoderm has been described as forming splanchnic and somatic 

 layers which unite with one another toward the median line. Where the 

 layers come together they are greatly thickened, and the thickened portion, 

 by a series of transverse constrictions, becomes cut into block-like masses 

 called mesodermic segments (proto vertebrae) . They are paired structures 

 bordering upon the medullary tube and increasing in number by the forma- 

 tion of new segments, chiefly posteriorly. A portion of them is seen on 

 the right of Fig. 2D, A; the rest have been removed. There is a longi- 

 tudinal depression separating the segments from the splanchnic and so- 



~ -^ _ ect. 



som 



FIG. 21. TRANSVERSE SECTION OF A 2.5 MM. HUMAN EMBRYO. (After von Lenhossek.) 

 (Compare this section with the upper part of the diagram, Fig. 19, I.) 



Ao., aorta; coe., coelom; ect., ectoderm; ent., entoderm; int., intestinal cavity; med. t., medullary tube; 

 nch., notochord; nepb., nephrotome; seg., mesodermic segment; som., somatic mesoderm; spl., 

 splanchnic mesoderm. 



matic layers, and the part of mesoderm which crosses the depression is 

 called the intermediate cell mass, or nephrotome. The coelom at first 

 extends through the nephrotome into the segments, as shown in the cross 

 section, Fig. 19, I. Later the segments and nephrotome become separated 

 from the lateral layers and from each other, and lose their cavities. This 

 has occurred in the nephrotome of Fig. 21. From the cells of the segments 

 the voluntary, striated muscles are derived, and from the nephrotomes 

 come the lining layer of the genital and urinary ducts and kidneys. From 

 all parts of the mesoderm certain cells become detached, and then unite 

 with one another by branching protoplasmic processes. Thus they form 

 a network, in the meshes of which is a clear intercellular fluid. Such 



