DEVELOPMENT OF SUBOCULAR LYMPH SACS 25 



of the sacs. These five sections are all of the steelhead trout and 

 it is unnecessary to state that the corresponding stages can also 

 be found in brook and rainbow trout embryos. Figures 19, 20, 

 22 and 23 are photomicrographs of sections of injected embryos 

 which were developed at a temperature of about 10.5°C (Series 

 II, 1913) ; figure 21 is a section of another steelhead trout embryo 

 developed at a still lower temperature and belonging to another 

 series (Series 1912). 



The very earliest indication of the anlagen of- the subocular 

 lymph sacs, as met with in the trout embryo, is shown in figure 

 19. These anlagen (1) appear as clear areas within a denser 

 mesenchyme and lie lateral to the carotid arteries (22) and 

 slightly caudal and ventro-medial to the eyes. The area occupied 

 by these anlagen is included within the leaders in the figure. 

 The section represented by figure 19 was cut in the frontal plane. 



So much variation occurs in the rate at which the subocular 

 lymph sac of the trout develops, that one is actually able to fol- 

 low every gradation in its development, from the time of its first 

 appearance until it has become a well-defined sac, even within a 

 single group of embryos of exactly the same age which have de- 

 veloped together under similar conditions of temperature. For 

 example, among twelve steelhead trout embryos (Series II, 1913) 

 developed in the same tank and which were killed on the four- 

 teenth day after fertilization, the following observations were 

 made : In two embryos no indication of the anlagen of the suboc- 

 ular sacs could be observed; in one embryo the anlagen of the 

 sacs (1) were represented as clear areas in the mesenchyme as 

 shown in figure 19; in three embryos the anlagen of the sacs were 

 clearly defined spaces in the mesenchyme as shown in figure 20 

 and, in each case, were included in one section; in one embryo 

 both sacs were included in two sections; in one embryo the sac 

 was included in two sections on one side of the body and in three 

 on the other; in one embryo the sac was included in two sections 

 on one side of the embryo and in five on the other ; in one embryo 

 the sac was included in three sections on one side of the body 

 and in four on the other; in one embryo the sac was included in 

 five sections on one side of the body and in four on the other, 



