30 DEVELOPMENT OF THE SYSTEMIC LYMPHATIC VESSELS 



reconstructions herewith published clearly show, to trace the 

 forming lymphatic with its atrophied vein kernel for long stretches, 

 and in different embryos of approximately the same crown-rump 

 measure the consistent repetition of identical histological pictures 

 is remarkable. 



There are, of course, as in the ontogeny of other structures, 

 individual cases of variation in which systemic lymphatic develop- 

 ment is either more advanced or more retarded than is normal 

 for the average of any given stage. But if a large number of 

 embryos of each typical period are examined and compared, the 

 average standard of extraintimal lymphatic development attained 

 by the majority of individuals in each stage is remarkably con- 

 stant and uniform. 



The earliest stage in which I have encountered this typical 

 replacement of an early embryonic vein by a perivenous extrainti- 

 mal lymphatic is presented by certain 12 mm. cat embryos along 

 the caudal circumference of the azygos-precardinal confluence. 

 In the concavity of the azygos arch on each side, as this vessel 

 turns ventrad to join the precardinal vein, these earliest evidences 

 of the extraintimal replacement of preceding embryonic veins 

 by independent perivenous lymphatic spaces are encountered. 

 Thus, fig. 10 shows a section of this region in a 12 mm. cat 

 embryo (series 217, slide x, section 12). Here the typical pic- 

 ture of the central collapsed core of the earlier vein (Jf), enveloped 

 by the clear extraintimal lymphatic space (5), is plainly to be seen. 

 Fig. 11 shows the same region in another 12 mm. embryo (series 

 211, slide x, section 15). Both the degenerating venule (4) 

 and the enveloping lymphatic (5) are larger than in the pre- 

 ceding series, and occupy the same position between aorta (7) 

 and left precardinal vein (6). 



In a 14 mm. embryo (series 127, slide viii, section 12, fig. 12) 

 the section passing just caudad to the junction of left azygos 

 (6) with left precardinal vein (6} shows these early lymphatic 

 spaces (5} and their relation to the contained venous remnant (4) 

 fully developed. Only one of the areas is denoted by leaders 

 in the figure, but two equivalent areas are seen further dorsad 

 and nearer to the ventral aspect of the azygos arch. The lym- 



