56 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



it increasingly turns to the left and continues to develop cephalad 

 along the ventral surface of the inter-azygos plexus, by extra- 

 intimal replacement of elements belonging to the latter. (Com- 

 pare the series of six reconstructions shown in figs. 188, 189, 

 190, 191, 192, 193.) 



The details of the adult organization of the thoracic ducts, and 

 the genesis of the observed adult variations on the basis of the 

 development of the embryonic channels, will be considered subse- 

 quently (Part V). The present communication is confined to 

 the consideration of the development of the preazygos and 

 azygos segments of the ducts. The thoracic ducts develop in cat 

 embryos between 11 and 16 mm. crown rump measure. In the 

 16 mm. embryo, McClure (l3 ' states that they "for the first 

 time make their appearance as definite, continuous lymph chan- 

 nels, which are disconnected from the venous system, except 

 through the mediation of the jugular lymph sacs." This is the 

 case in the majority of 16 mm. embryos, but in a number of 15, 

 15 5 and 16 mm. series in my collection the final union of the 

 individual segments has not yet taken place. It is possible, by 

 combining a large number of observations on numerous embryos 

 of the same length, to establish an average developmental condi- 

 tion of the thoracic ducts for each stage in embryos between 12 

 mm. and 17 mm. crown-rump measure. Individual embryos 

 .will, however, show marked differences in development, and 

 many departures from the standard plan are encountered. 

 The same conditions obtain here which McClure and I found in 

 our study of the development of the jugular lymph sacs. Some 

 embryos show, for their length, unusually advanced thoracic 

 duct development in certain areas, while in other parts this 

 development is relatively retarded and more primitive. In other 

 series the entire region of thoracic duct development appears 

 more uniformly ahead of the average condition, normally en- 

 countered in embryos of this period. Finally, in a third group, 

 the appearance of the typical lymphatic development seems to 

 be delayed until a later stage than is normally the case. But in 

 spite of these individual differences, a uniform and consistent 

 ground plan of thoracic duct development can be demonstrated, to 



