6 DEVELOPMENT OF LYMPHATIC SYSTEM, FISHES 



II. THE EMBRYONIC GROUND-PLAN OF THE LYMPHATIC SYSTEM 

 IN THE TROUT UPON WHICH THAT OF THE ADULT IS BUILT 



The time at which the main lymphatic channels are first met 

 with in the embryo in the form of a continuous system of ves- 

 sels, naturally varies in relation to the temperature of the water 

 in which development has taken place. It is therefore impossible 

 to state definitely for any one of the species studied, that a 

 system of continuous lymphatic channels will be uniformly met 

 with on any particular day. In the rainbow and steelhead trout, 

 when developed at an average temperature of about 10.5C., a 

 continuous system of lymphatic channels was found to be present 

 on both sides of the head and pharynx on the twenty-second day 

 after fertilization in fully 90 per cent, and, on the twenty-third 

 day, in practically all of the embryos injected. In another series 

 of steelhead trout embryos developed at an average temperature 

 of 11.5C, a continuous system of lymphatic channels was met 

 with on both sides of the embryo on the twentieth day after fer- 

 tilization in practically all of the embryos injected. 



Figure 10 represents a reconstruction of the principal arteries, 

 veins and lymphatics (main trunk lines) found in the regions of 

 the head and pharynx of a rainbow trout embryo on the twenty- 

 second day after fertilization. It represents the earliest condition 

 met with in the trout embryo in which the principal or main 

 lymphatic channels of the head and pharynx form, for the most 

 part, a continuous system of vessels which drain into the veins 

 at typical points. This particular reconstruction may be said to 

 represent the ground-plan arrangement of the embryonic lymphatic 

 system upon which that of the adult trout is built. It was recon- 

 structed from sections of an uninjected embryo in order to em- 

 phasize the ease and accuracy with which the lumina of the 

 lymphatics can be followed in sections, without the aid of in- 

 jections, after continuous vessels have been established. The 

 principal lymphatic channels are as follows: 



1. The lateral pharyngeal lymphatic. 



2. The subocular lymph sacs. 



3. The medial pharyngeal lymphatic. 



4. The precardinal or jugular lymphatics. 



