456 EMBKYOLOGY OF THE LOWER VERTEBRATKS < ,,. 



spicuous. Tlie zona radiata is usually looked upon as primary in 

 its nature but this is by no means settled and some cnmprtent 

 authorities regard it as secondary. 



Outside the zona radiata there may often be found a second 

 envelope which does not show the perforations characteristic of the 

 zona radiata: this also in the case of the large heavily yolkrd 

 eggs becomes thinned out during the process of growth. Envelopes 

 of this type are specially conspicuous in those Vertebrates in 

 which there is no great development of tertiary envelopes secreted 

 by the oviducal wall, e.g. Teleostean fishes. In such cases the 



FIG. 206. A, cluster of eggs of lidellostoma,' attached together by the interlocking of tln-ir 

 anchoring filaments ; B, apical portion of the egg-shell, showing the anchoring lilanu-nts 

 projecting from tin- middle of the separable "lid." (Figure by lJ:ishf..rd Dean, 1'iom 

 The (Iniiliridy .\<ifnr<tl History.} 



outer layer of tin- envelope, lying immediately external to the 

 typical radiate layer, frequently shows characteristic modifica- 

 ti-iiis, consisting of closely packed villi or columns composed of cells 

 \vhidi swell up and become strongly adhesive, serving to attach the 

 eggs to one another or to a solid object (Roach Lemiscus rutilus, 

 Bleak Alburnus, Herring Clupea harenyus). In the Actino- 

 pi.-ry;jian (ianoids a similar condition is found. In Ceratodu* this 

 serial outer layar 1C not lound, there being here a tertiary envelope 

 of jelly. In Lepidosiren and J'mfopterus the tertiary envelnpe is as 

 a nil.- no lunger tornn-d, t,hc J-M-O-S 1\ in- I.M.S,- in the Indium nf the 

 burrow, though it, is ol' interest to notice that in /^/m/</.s//r// the 

 'ion nf a jelly like tci't.iai-y i-n \ fl ijir round the egg s is still 



tonally found as an individual yariation. In .Petromyzon, a- in 



lh- TeleostH alludeil to above, a radiate .-nvelnp,- is luiind inside a 

 One which hecoinrs s\\ollcn up and Micky on tin- 



