The Origin and Development of the Lymphatic System. 17 



from the deeper coarse-meshed or subcutaneous plexus, and, since the 

 secondary plexus is growing actively, it shows many sprouts. 



The' proof of growth by sprouting from injections is always an 

 interpretation of appearances, and though it is a logical inference, 

 one must turn to the observations of the living form for conclusive 

 proof. This has been given by E. E. Clark (25-26), who has again 

 restudied the classical object, the tadpole's tail. The introduction 

 of chloreton anaesthesia is one of the factors that have made this study 

 possible, since the same specimen can be kept alive for weeks, notwith- 

 standing repeated doses. Dr. Clark's devise of an upright chamber, 

 so arranged that the tadpole can be kept upright without the pressure 

 of the cover slip, and the finding of larva of the form Hyla picker- 

 ingii in the spring of 1910, a form which has few pigment cells, have 

 all combined to make his studies so valuable and convincing. He 

 has described not only the complete history of a lymphatic capillary, 

 watched through periods of weeks, but has followed every connective 

 tissue cell in the neighborhood (30) through several generations. Dr. 

 Clark has shown that the wall of the lymphatic capillary is in ceaseless 

 activity. In the living form the wall is in part hyaline, in part granu- 

 lar. The nuclei are surrounded and obscured by granular protoplasm, 

 so that they are clearly seen only when dividing. The wall is of 

 irregular thickness, often extremely delicate, and from the sides and 

 tips are sent out numerous tiny processes, some hyaline and some 

 granular, which may be well described as amoeboid. A few of these 

 persist and grow into permanent lymphatics. Most of them are with- 

 drawn. These tiny sprouts, indicating the functional activity of the 

 vessel, do not disappear in alcoholic specimens (26, p. 403). They 

 are at first without nuclei, but nuclei wander into them from the 

 parent stem; indeed, two nuclei may pass each other as they advance 

 or recede. This proves that the growing amceboid wall of the lym- 

 phatic capillary is a syncytium and explains the failure to obtain the 

 silver markings at the growing tip. The silver markings indicate 

 the more stable lymphatic capillary. 



E. E. Clark's most recent observations (30) show that increased 

 activity of the lymphatic wall in the sending out of many of the tiny 

 processes is a sign of growth, so that an area can thus be selected to 

 watch the processes of the formation 'of new vessels. 



As far as the relation to the surrounding connective cells is con- 

 cerned, the growing, and, we might also say, the functioning, amoeboid 

 tips avoid the cell bodies and processes of the mesenchyme cells. 



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