STUDY OF SECTIONS OF EMBRYO OF 20 MM. 315 



lymphatic spaces, somewhat irregular in form, and lined by a thin endothelium so 

 that they present a close resemblance to veins in their structure; if followed up 

 toward the head the lymphatics are found to unite with the large jugular lymph- 

 sac (Fig. 60 , s.l.f). Close to the medial wall of the jugular vein is situated the 

 large trunk of the vagus nerve, N.io. 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 somewhat conspicuous. The cells of the thyroid form 

 an irregularly shaped branching mass. The spaces between the branches are chiefly 

 occupied by small endothelial blood-vessels. 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 com- 

 pactly arranged without distinct cell-boundaries, but with protoplasm which stains 

 somewhat and with nuclei of rounded form, distinct outline, and granular appear- 

 ance, the granules being decidedly more conspicuous than the granules in the nu- 

 clei of the neighboring mesenchymal cells. Just ventral to each jugular vein is 

 a small darker body, consisting of closely compacted cells, resembling in appear- 

 ance 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 mesen- 

 chyma 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 section is an indication of the anterior 

 limb, A.L, and of its vein, Ve". 



Section through the Lungs (Fig. 214). 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 208, 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. Im- 

 mediately around the notochord the spaces occupied by the cells are the largest, 

 the capsules most distinct, and the nuclei most altered. Proceeding toward the 

 periphery of the cartilage, the cells appear in successively earlier and earlier stages, 

 until at the very periphery we have normal nuclei and a transition to mesenchyma. 

 The notochord has contracted, leaving a space between the notochordal cells and the 

 vertebral cartilage. Immediately below the vertebra are the conspicuous anlages 

 of the sympathetic system, Sym. They overlie the sections of the posterior cardinal 



