OVARIES IN TELEOSTEAN AND ELA8M0BRANCH FISHES. 185 



I'ests immediately on the surface of the yolk {vt.). In the 

 egg under consideration most of the yolk spheres have flowed 

 together into a single mass occupying the bulk of the egg. 

 Only in the cortical protoplasmic layer are separate sphei'es 

 and oil globules still to be found. In this preparation the"* 

 egg membrane appears to be homogeneous, but in other sec- 

 tions there are indications of a fibrillar structui-e. On surface 

 view the membrane is distinctly punctate. 



Fig. 17 represents an earlier condition of the egg mem- 

 branes, etc. This figure was drawn from a section of an egg 

 •5 mm. in diameter, in which numerous oil globules but no 

 yolk spheres were present. Here the follicular epithelium, 

 as before, is a single layer of cells. The egg membrane {z.) 

 is very thin, no thicker than the membrana propria, and 

 reposes on a peculiar layer consisting of cytoplasmic threads 

 closely set and radially arranged with reference to the centre 

 of the egg. The spaces between tlie threads do not appear 

 to contain any substance. If any substance is present it is 

 structureless, nnstainable, and therefore invisible. This 

 layer is tlie zonoid layer observed by His, Scharff (1887), 

 and others in various teleostean ova. At this stage the 

 zonoid layer is clearly delimited from the subjacent cytoplasm 

 by what appears as a rather thin line under moderate powers 

 of the microscope ; it is probably caused by a condensation 

 and parallel arrangement of cytoplasmic trabeculte. In still 

 smaller eggs, in which the protoplasm is denser and more 

 compact, the radial fibrils composing the zonoid layer are 

 still more closely arranged, and are directly continuous with 

 those of the general network ; that is to say, there is no line 

 of separation on the side of the vitellus, the zonoid layer 

 appearing rather as a differentiation of the superficial proto- 

 plasm. On the side of the egg membrane the zonoid layer is 

 likewise bounded by a thin surface distinct from the inner 

 boundary of the egg membrane proper, but this can only be 

 seen in sections in which, owing to the contracting effects of 

 reagents, the zonoid layer has parted from the egg membrane 

 proper. 



