STUDY OF SECTIONS OF EMBRYO OF 17 MM. 



303 



through in the series of sections, it may be found that the condensed mesenchyma 

 surrounds the notochord, which, therefore, passes through the central portion of 

 each intervertebral anlage. The bodies of the vertebrae at this stage consist merely 

 of the loose mesenchyma between the intervertebral discs, are entirely without any 

 distinct limitation, and merge into the surrounding loose mesenchyma. Near the 

 anterior border of each nerve-trunk, and usually somewhat toward the median 

 side of it, lie -the intersegmental vessels, which are of small size and vary greatly 

 in their exact position and number, according as they are more or less branched. 

 Between the ends of the vertebral bows outside of the nerve-trunks can be seen 

 with higher power clusters of elongated cells with developing muscle-fibers which 

 are here still segmentally arranged between the processes of the developing 

 vertebrae. 



Pig Embryo of 17 mm. Study of Sections. 



Since the pig of 12 mm. contains the anlages of perhaps every important 

 part of the body sufficiently advanced in development to be clearly recognized, 

 we find in the immediate subsequent development that we have to do not so 

 much with an introduction of new parts as with the differentiation of those which 

 have already commenced. Embryos of 17 mm. are convenient for the study of 

 the differentiations referred to. Particularly important for the student to note 

 are the advances in the development of the vertebrae, of the lungs, of the Wolffian- 

 bodies and genital glands, and of the kidneys. These points are illustrated in 

 figures 208 to 210, representing portions of three transverse sections of a 17 mm. 

 embryo. 



Transverse Section through the Lungs (Fig. 208). The epidermis of the embryo 

 %as become more distinct owing to its growth in thickness, which is .ompii. hed 

 by the increase of the number of layers of cells. The growth is very marked a l 

 the sides of the section about the level of the vertebra. At these points it ca 

 early seen that upon the outside the epidermis has a very thin layer of flattened 

 the nuclei of which are themselves also somewhat flattened. This single 

 jf cells is known as the epitrichium, because the hairs are developed en- 

 rneath it. Where the epidermis is thickest, one can observe that the 



layers 

 They 

 Between 

 forming the i 

 developed and 

 carries the nerves 

 of more darkly staine 

 cells proper, the anlage 

 thelial muscle-cells, 



to the mesoderm are closely packed together with round nuclei. 

 commencing formation of the basal layer of the adult epidermis. 

 layer and the epitrichium the cells are more loosely placed, 

 of the mucous layer. The mesenchyma is very much 

 a large territory in the dorsal region of the embryo. It 

 blood-vessels and shows at various points accumulations 

 are of two kinds: first, groups of mesenchymal 

 the skeleton; and, second, groups of meso- 

 skeletal muscles. There is little 



differentiation otherwise 



may note the following changes 



