247 



spherules are sucked back it may remain protruding from the rupture. 

 In other cases it may be traced within the zona. In the living fertili- 

 sed eggs of Pleuronectes flesus I was able to trace it up over the edge 

 of the segmenting blastodisc ; in some, after death, when the cortical 

 protoplasm began to retract, the delicate membrane was carried Avith 

 it and was thrown into wrinkles in the neighbourhood of the blastodisc. 

 The membrane therefore continues to cover the vitellus after the peri- 

 viteiline space is formed. 



Immediately beneath this membrane is the thin cortical layer 

 of protoplasm or periblast, most readily seen in mounted sections, with 

 the minute shining granules or vesicles imbedded in it. These resist 

 stains, dilute acetic acid, alcohol and ether; they persist in the mature 

 eggs for a long time after death, even when the cortical larger has 

 shrunk and partially disintegrated; and (apart from oil globules when 

 these are present) they are the only definite bodies which can be re- 

 cognised to continue unchanged within the pelagic egg throughout 

 the stage of maturation. The cortical protoplasm is continuous inter- 

 nally with the scanty matrix between the yolk -spherules which 

 make up the great mass of the e^^. The yolk-spherules in pelagic 

 eggs differ from those in demersal eggs in being smaller and less 

 dense. The largest measure about 0,02 mm; they may be clear or 

 may contain fine granules ; after treatment with osmic acid they show 

 concentric lamination. In demersal eggs the spherules may reach 

 -0,05 mm [Zeus fahei') or 0,07 mm [Gohius iiiger, Lophius piscatorius]'^ 

 they are markedly granular or may be made up of solid-looking ré- 

 fringent bodies, presenting sometimes a mulberry-like appearance. In 

 eggs possessing in the mature condition one or more oil globules, 

 numerous oil-droplets are arranged around the germinal vesicle, which 

 fuse together and collect on the upper surface of the yolk when the 

 watery fluid enters. 



The germinal vesicle is central, typically spherical, and relatively 

 large, occupying from about one-fourth to nearly one- third of the 

 diameter of the ovum. It can readily be isolated from the living egg; it 

 is soft and plastic, with a distinct membrane, finely dotted on the sur- 

 face (probably from rupture of threads of linin) and it contains nume- 

 rous , conspicuous, réfringent nucleoli, arranged more or less regularly 

 around the periphery, and which stain deeply. The chromatin net- 

 work cannot well be made out except in the young eggs. 



The structure of the fully matured pelagic egg, ready for fertili- 

 sation, is simple. It consists of the envelopes described, the cortical 

 layer of protoplasm (which may in some forms become aggregated at 

 one pole to form the blastodisc) and a homogeneous transparent fluid 



