184 WrLLIAM WALLACE. 



of egg protoplasm is free of yolk. (2) No yolk granules are 

 ever found in the cells of the follicular epithelium unless the 

 egg is undergoing absorption through the action of these 

 cells. In the normal follicle, while deposition of yolk is 

 going on in the egg, no granules are seen in the follicular 

 cells. 



Caldwell (1887), speaking of the ovarian egg of Echidna, 

 states that " food material is formed in cells of the follicular 

 epithelium as well as in the ovum itself, and appears first as 

 minute granules in the neighbourhood of the nuclei and 

 germinal vesicle, and every stage is to be found, from the 

 most minute granules up to the largest yolk spheres of the 

 mature ovum. From the nucleus of each follicular cell, and 

 from the germinal vesicle, streams of these yolk granules 

 travel into the body of the ovum. Further, "the streams of 

 yolk granules passing into the ovum, immediately on enter- 

 ino-, ai'C more or less parallel to each other, and give a 

 radially striated appearance to the peripheral layer of the 

 ovum. A similar appearance in other vertebrate eggs has 

 been described as a definite membrane, and named the zona 

 radiata." This is doubtless the zonoid layer again. If 

 so, I cannot agree with Caldwell's explanation of it. The 

 above observations of Caldwell on the nutrition of the egg in 

 Echidna were, it is necessary to remember, made on fixed, 

 stained, and sectioned ova. 



In fig. 18 the investments of a nearly ripe ovum of 

 Zoarces ai*e shown in section. The material sectioned had 

 been previously fixed in Bles's fluid and then stained with 

 borax carmine and picro-nigrosin. The section is that of an 

 almost ripe egg 2*5 mm. in diameter. The egg membrane 

 measured '004 mm. in thickness. The follicular epithelium is 

 separated on the outside from the thcca folliculi by a fine 

 homogeneous membrane, themembrana propria folliculi. 

 The follicular epithelium consists of a single layer of flat cells 

 with elliptical nuclei. These nuclei lie with their long axes 

 parallel to the surface of the egg. The cells repose directly 

 upon the outer surface of the egg membrane, and the latter 



