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. 



From a genetic standpoint the lymphatics can be spoken of as those derived 

 directly from veins and those formed from intercellular spaces in the mesen- 

 chyme. The lymphatics derived d rectly from veins are confined to clearly 



Ant. cardinal vein 



Right 

 lymphatic duct 

 Subclavian vein 



Post, cardinal vein 



Mesonephro: 

 Kidney 



Sciatic vein' 

 Femoral vein ; 



Ant. lymph heart 



Thoracic duct 



" Post, lymph heart 



Fig. 252. — Diagram showing the arrangement of the lymphatic vessels 

 in a pig embryo of 20 mm. Sabin. 



circumscribed regions of the body, namely, the region of certain tributaries of 

 the precardinal and postcardinal veins on each side, and are spoken of as the 

 lymph sacs or lymph hearts. The lymphatics formed from intercullar spaces 

 are distributed generally throughout the body and represent not only the larger 

 lymphatic trunks, apart from the sacs or hearts mentioned above, but also 

 all the finer ramifications. These independently formed lymphatic vessels 

 are spoken of as systemic lymphatics in contradistinction to the sacs or hearts 

 which are of venous origin. 



