402 OTTO F. KAMPMEIER 



and elongate with the increasing age of these embryos; in other 

 words, in a 22 mm. embryo the blind segments of the duct anlage 

 will be much longer and more conspicuous than in a 20 mm. em- 

 bryo for instance. Further, there is a positive regularity in the 

 progressive reduction of the number of these blind lymphatic 

 anlagen in a general antero-posterior direction by their addition 

 to the continuous anlage, which, as a consequence, gradually 

 becomes elongated. Were these lymphatic spaces artifacts, or 

 segments cut off from a continuous channel by shrinkage, then 

 the determinate sequence of genetic changes pointed out in the 

 descriptions of the individual stages could not exist, and we should 

 find them in slightly older embryos or in those portions of the 

 duct-anlage definitely known to be complete, for the same methods 

 of technic should produce similar effects. The embryonic tho- 

 racic ducts when fully formed and indeed all lymphatic vessels 

 possess a varicose channel constricted and dilated alternately into 

 irregular nodes and internodes. Such a condition, however, is 

 not brought about by fixation but is a characteristic peculiar 

 to a lymph vessel and obviously harks back to the period when it 

 was composed of a varying number of irregular fusiform or oblong 

 mesenchymal spaces succeeding one another with distinct inter- 

 ruptions. Accordingly, the nodes or constrictions of a thoracic 

 duct just completed would indicate the areas of final fusion 

 between consecutive anlagen. 



It should be emphasized here that Sabin and Clark base their 

 criticisms chiefly upon the latter's investigations on the develop- 

 ment of the lymphatic capillaries in the tail fin of the larval frog. 

 The fallibility of their argument becomes therefore further evi- 

 dent when we find them comparing the reaction to the fixatives 

 of these terminal lymphatics with that of other lymph channels, 

 especially the larger ducts and trunks; for, although the principle 

 of development probably is the same in both cases, the details 

 of their behavior during the preparation of the sections may be 

 quite different. It would be just as logical to describe a large 

 systemic artery or vein entirely in terms of their terminal arterioles 

 or venules. The writer will not deny that careless or imperfect 

 fixation may cause the delicate capillars s of the fin of a tadpole 



