276 



TEXT-BOOK OF EMBRYOLOGY. 



sometimes be seen apparently extruding globular elements which simulate the 

 plates in appearance and staining reaction. 3. The view that the plates are 

 independent nucleated bodies has received support from the fact that they 

 possess faint chromatic masses after treatment with certain dyes, and that they 

 possess the power of amoeboid movement. 4. The recent view (Wright) that 

 " the blood plates are detached portions of the cytoplasm of those giant cells of 

 the bone marrow and spleen, which have been named megakaryocytes by Howell " 

 is strongly supported by direct observation. 



THE LYMPHATIC SYSTEM. 



The Lymphatic Vessels. 



Researches have led to two general views concerning the origin and develop- 

 ment of the lymphatic vessels. One view * is that the first lymphatic channels 

 are enlarged spaces in the mesenchyme. Certain spaces which exist among the 



Ant. cardinal vein 



Right 



lymphatic duct f 

 Subclavian vein \ 



Post, cardinal vein 



Mesonephros- 

 Kidney 



Sciatic vein 

 Femoral vein 



Ant. lymph heart 



Thoracic duct 



V"T Post, lymph heart 



FIG. 252. Diagram showing the arrangement of the lymphatic vessels 

 in a pig embryo of 20 mm. Sabin. 



mesenchymal cells enlarge, while the cells bounding these spaces become dif- 

 ferentiated into the endothelial lining of the channels. The spaces or lacunae 

 thus developed become confluent and form a network of vessels which, by the 

 same process, gradually extend throughout the body and constitute the vast 

 system of lymphatics. These open into the veins secondarily. The first points 

 of origin for the system are found in the axilla and groin during the second 

 month in the human embryo (Fig. 251). 

 *Gwlland, Sala. 



