DEVELOPMENT OF SUBOCULAR LYMPH SACS 31 



since, as already demonstrated by figures 1, 2 and 25, the sub- 

 ocular lymph sacs develop at first remote from the infraorbital 

 veins (23) and only subsequently come into contact with them in 

 the course of their forward extension. Figure 24 is a section 

 taken through the subocular lymph sacs of a sixteen-day rainbow 

 trout embryo in the region where the sacs (1) and the infraorbital 

 veins (23) are in close contact with each other (compare figures 

 24 and 25). Although the infraorbital veins in the sixteen-day 

 trout embryo have been injected to their fullest extent, it is 

 seen that the subocular lymph sacs have not received any of the 

 injecta: This has been found invariably to be the case in all 

 trout embryos which have been inj ected. When blood corpuscles 

 are met with in the subocular lymph sacs, in stages where the 

 latter are still independent, their presence must therefore be 

 attributed to other causes than to a connection between these 

 sacs and the veins. Although never abundant, such corpuscles 

 are, however, sometimes met with and, if they have not been 

 extravasated into the sacs from the veins, or floated over into 

 them in the balsam, they must have been derived from the walls 

 of the sacs, or taken up from the adjacent mesenchyme. 



The method by which the caliber of the subocular lymph sac 

 increases in size and by which the original anlage of the sac ex- 

 tends in an anterior direction, are questions of considerable 

 interest and importance. 



Soon after the subocular lymph sac of the trout has been estab- 

 lished in the form of a single-chambered structure, it rapidly 

 increases in caliber. This appears to take place by division of the 

 cells which form its walls and not by the further addition to it of 

 mesenchymal spaces. In other words, the increase in caliber or 

 expansion of the sac along its transverse and dorso- ventral diame- 

 ters, results from a growth of the walls of the sac and, in this 

 respect, resembles the embryonic aorta which, after it has once 

 been laid down, gradually increases in caliber in this same 

 manner with the growth of the embryo. 



The extension of the subocular sac in an anterior direction 

 does not appear to take place, however, in exactly the same 

 manner. Sections taken through the anterior end of the sub- 



