DEVELOPMENT OF STJBOCULAR LYMPH SACS 27 



the subocular lymph sacs of the trout embryo from the veins 

 prior to a relatively late stage of development, when a connection 

 is then secondarily established between the sacs and the lateral 

 pharyngeal lymphatic. It can be definitely stated, therefore, 

 that at the time of their inception and during the early stages of 

 their development, the subocular lymph sacs of the trout embryo 

 do not communicate with the veins. Since the anlagen of the 

 subocular lymph sacs cannot be injected from the veins, it is 

 evident that the injection method neither reveals their presence 

 in the living embryo, nor does it throw any light upon their actual 

 mode of origin. 



Do the anlagen of the subocular lymph sacs of the trout 

 embryo at first possess an endothelium? 



As shown in figures 19 and 20, the earliest anlagen of the 

 subocular lymph sacs (1) appear as clear areas in the mesen- 

 chyme which are bounded on all sides by more closely arranged 

 mesenchymal cells. Figures 1 and 2 are reconstructions of the 

 left and right sides, respectively, of the embryo from which 

 figure 20 is taken, and they show the position and relative size 

 of the anlagen of the subocular lymph sacs (1) at about the time 

 of their first appearance in the embryo. The anlage of the sac 

 shown on the left side of figures 19 and 20 is more clearly defined 

 in section than that on the right side. An examination of these 

 clear areas (1) under high magnification' shows that they have 

 not been injected and that the clear appearance they present is 

 due to the presence of a loosely arranged mesenchyme, between 

 the strands of which intercellular lymph spaces are present. As 

 these intercellular lymph spaces gradually increase in size, the 

 anlagen of the subocular lymph sacs often present a multilocular 

 appearance (fig. 21), due to the presence of mesenchymal strands 

 or bridges which extend between their walls and divide the anlage 

 of the sac into a number of compartments. These bridges rap- 

 idly break down, however, so that a single-chambered sac finally 

 results (fig. 22) . The presence of nuclei within the anlagen of the 

 subocular sacs (left side, figs. 19 and 20), is a feature quite 

 characteristic of developing lymph vesicles found elsewhere in the 

 embryo. Such nuclei often appear as isolated bodies in an other- 



