RELATION OF LYMPHATIC TO BLOOD-VASCULAR SYSTEM 51 



These early lymphatic anlages, whether formed independently 

 in mesoderm, or on the site of phylogenetically abandoned venous 

 lines, or, as is generally the case throughout the body, in close 

 correlation to the embryonic venous pathways, always appear 

 in the same situations and, in the average embryo, at the same 

 developmental period. Their constant character, and regular 

 occurrence and relations, repeated within very narrow limits of 

 individual variation in every embryo of the proper stage, imparts 

 to them a definite morphological character. In every series of the 

 proper age in my collection I find the same spaces in the same 

 place and in identical relationship to the adjacent veins. In some 

 individuals, as above stated, they develop earlier than usual, in 

 others their appearance is retarded, but this applies only to the 

 achievement of the full development tyj^ical for the average embryo 

 of a given stage. In the retarded individuals the same structures 

 are always present, only they are less strikingly developed and less 

 numerous when compared with the average normal type charac- 

 teristic of the period under consideration. With sufficient mag- 

 nification it is not difficult to distinguish sharply between the 

 perihaemal lymphatic spaces and the blood-vascular channels 

 proper. 



With sufficient material every stage of their development can 

 be followed up to the confluence of the entire system and its final 

 entrance into the jugular lymph sacs. 



These spaces are neither artefacts due to embryonic shrinkage, 

 nor are they the unfilled portions of the blood-vascular capillary 

 network. They are, on the other hand, the well-defined earliest 

 anlages of the systemic lymphatic vessels. The more perfect, as 

 a matter of fact, the embryonic fixation is, the more clearly can 

 these structures be indentified under the microscope. Their 

 history, as will be shown presently, can be traced with the utmost 

 accuracy, and they can be followed step by step in their de- 

 velopment up to their inclusion in the completed and connected 

 lymphatic channel system. 



But even in their earliest stages they possess an unmistakable 

 and definite morphological character, quite as" distinct as that 

 of the adjacent blood channels. They can be followed closely in 



