1917 Riddle J on Chinook Salmon 321 



The zona radiata is a tough, elastic, semi-opaque membrane which 

 covers the egg. In the specimen examined it was O.OGi? mm. thick. It 

 contained canals similar to those described in the zona radiata of the trout 

 by Henneguy (22). It is not adhesive like some of the egg membranes 

 of the pelagic teleostean ova. A space exists below the zona radiata, leav- 

 ing the egg enclosed by it free to move within. The disk of the living egg 

 is always found uppermost, no matter in what position the egg is placed. 

 The cavity is exaggerated in preserved eggs by the shrinkage of the yolk. 

 The bulk of the egg is composed of an albuminous yolk {Fig. 2) 

 which is a transparent fluid in the living egg but is coagulated in the 

 preserved egg into a transparent, hyaline mass. The albuminous yolk is 

 nomogeneous thruout and as far as could be determined has no structure. 

 Resting on the top of the yolk and embedded in it is a cap of globules 

 (Fig. 2), This cap is about 4 mm. in diameter and from 1 to 2 mm. deep 

 at the center. The globules in the living egg are spherical but are slightly 

 distorted by preservation. The content of these globules is dissolved out 

 in the pre-ovum, leaving vacuoles in the yolk (Fig. 4). These vary con- 

 siderably in size. Those immediately beneath the disk are smaller than 

 the ones farther from it (Figs. 3 and 4). They are also found else- 

 where in a single layer on the surface of the yolk (Fig. 3). 



These drops give the rosy color to the egg and resemble the globules 

 in the tissues of the salmon that are mentioned by Greene (19). Accord- 

 ing to Henneguy (22), the content of these globules is some sort of oil. 

 Prince (34) suggests that there is also lecithin in the content of the drops. 

 Their function is not clear, although they persist in the yolk sac after 

 hatching and are found in certain relations to the advancing germ ring. 

 They probably furnish the developing fry with nutrient substance. 



In the ovarian egg the granular substance is evenly distributed in a 

 thin layer over the entire surface of the egg (Fig. 2). In the egg taken 

 from the abdominal cavity of the female, the greater part of the germinal 

 material is concentrated in a disk on the upper pole and rests upon the 

 oil cap (Fig. 3). This disposition of the germinal substance is quite con- 

 trary to that in the s.ea bass (Wilson, 49) and the cod (Ryder, 36). In 

 these pelagic eggs the granular material is not concentrated until fertiliza- 

 tion is effected and then upon the lower side. The formation of the ger- 

 minal disk is described for the trout by Henneguy (22) and is like that 

 of the salmon as described above. The disk thus formed is lenticular in 

 section and fades out at the periphery into a thin layer which extends 

 superficially over the yolk. 



SEGMENTATION 



Owing to the opacity of the zona radiata, the delicate structure of 



