42 HISTOLOGY 



the blood and conveying them through the Wolffian duct to the bladder. 

 Later, parts of the urinary system lose their primary function and become 

 the ducts of the genital system. 



The lateral somatic and splanchnic layers of the mesoderm produce 

 the lining of the pleural, pericardial, and peritoneal subdivisions of the 

 ccelom, as already stated: They give rise also to an important tissue 

 known as mesenchyma. With the production of mesenchyma the tissues 



Epi. 



i.s. 



B.V 



FIG. 31. SECTION FROM THE HEAD OF A RABBIT EMBRYO OF ioj DAYS, 4.4 MM., TO SHOW MESENCHYMA. 



Epi. and M. T., Ectodermal epithelium of the epidermis and medullary tube, respectively. N., nucleus, 

 P., protoplasm, and I. S., intercellular substance of a mesenchymal cell. Two of these cells show 

 mitotic figures. B. V., Blood vessel, lined with endothelium. One of the blood vessels contains an 

 embryonic red blood corpuscle. 



of the embryo may be divided into two sorts, namely, epithelium which 

 covers an external or an internal surface of the body, and mesenchyma 

 which fills the space between two layers of epithelium. These relations 

 are clearly shown in the cross section of the abdomen (Fig. 30). The 

 body wall consists of a layer of ectodermal epithelium externally, and of 

 mesodermal epithelium internally, with a thick layer of mesenchyma 

 between the two. Similarly the intestinal wall consists of mesodermal 

 epithelium toward the ccelom, and entodermal epithelium toward the 

 intestine, with mesenchyma between them. Epithelium is thus pro- 

 duced by all the germ layers, but mesenchyma is almost exclusively the 

 product of the mesoderm. It is formed not only from the lateral splanchnic 



