244 STUDY OF PIG EMBRYOS. 



and characteristic young sympathetic nerve-cells, by which it is readily recog- 

 nized. Close to the ventral side of the sympathetic is the section of the large 

 jugular vein, V. jug, a branch of which, V. br, lies laterad from the main vessel. 

 This branch receives blood-vessels from the facial region, and is perhaps the facial 

 vein, but its identity is not certain. Between the main jugular and its branch 

 are some lymphatic spaces, somewhat irregular in form, and lined by a thin endo- 

 thelium so that they present a close resemblance to veins in their structure. Close 

 to the medial wall of the jugular vein is situated the large trunk of the vagus 

 nerve, N. to. At a little lower level than the vagus nerves and in the median line 

 lies the anlage of the thyroid gland, which, owing to its darker staining, is some- 

 what conspicuous. The cells of the thyroid form an irregularly shaped branch- 

 ing mass. The spaces between the branches are chiefly occupied by cavities 

 lined by endothelium and which probably belong to the lymphatic system. The 

 arrangement of these cavities and the relation of their endothelium to the cells of 

 the organ recall the blood sinusoids of the liver and of the suprarenal capsule. 

 The thyroid cells are compactly arranged without distinct cell-boundaries, but 

 with protoplasm which stains somewhat and with nuclei of rounded form, dis- 

 tinct outline, and granular appearance, the granules being decidedly more con- 

 spicuous than the granules in the nuclei of the neighboring mesenchymal cells 

 Just ventral to each jugular vein is a small darker body, consisting of closely 

 compacted cells, resembling in appearance those of the thyroid. The body has a 

 very distinct external outline and is actively growing, for several of its nuclei are 

 in mitosis. The bodies in question are the parathyroid glands. The rest of the 

 section is mainly occupied by mesenchyma and numerous darker masses, muse, 

 the anlages of the various muscles of the neck and throat. On each side is shown 

 a small piece of the cartilaginous scapula, Scap. At the lower corner of the sec- 

 tion is an indication of the anterior limb, A . L, and of its vein, Ve". 



Section through the Lungs (Fig. 142). — The spinal cord shows very clearly in the 

 differentiation of the three primary layers of the medullary wall. Its structure is 

 similar to that shown in figure 1 4 1 , and need not be again described. The vertebra, 

 Vert, is now distinctly young cartilage. On its ventral side its boundary is quite 

 distinct, the formation of the perichondrium having there begun. Laterally it 

 merges into a dense mesenchyma, by which it is united without demarcation with 

 the rib, cost', and indirectly with the vertebral arch, V. ar, both of which are 

 cartilaginous. The cells of the vertebral cartilage occupy rounded cavities, each 

 of which is marked by a distinct capsule. The matrix between the capsules is 

 homogeneous, stains slightly, and has acquired a greater density than in earlier 

 stages. The cells themselves exhibit traces of their protoplasmic bodies and 

 have deeply stained nuclei which are quite irregular in shape and very granular. 



