456 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



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

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

 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 yolked 

 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 



Pig. 206. — A, cluster of eggs of Bclellostoma, attached together by the interlocking of their 

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

 projecting from the middle of the separable " lid." (Figure by Bashford Dean, from 

 The Cambridge Natural- History.) 



outer layer of the envelope, lying immediately external to the 

 typical radiate layer, frequently shows characteristic modifica- 

 tions, consisting of closely packed villi or columns composed of cells 

 which swell up and become strongly adhesive, serving to attach the 

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

 Bleak — Albumus, Herring — Clupea harengus). In the Actino- 

 pterygian Ganoids a similar condition is found. In Ceratodus this 

 special outer layer is not found, there being here a tertiary envelope 

 of jelly. In Lepidosiren and Protopterus the tertiary envelope is as 

 a rule no longer formed, the eggs lying loose in the bottom of the 

 burrow, though it is of interest to notice that in Lepidosiren the 

 secretion of a jelly-like tertiary envelope round the eggs is still 

 occasionally found as an individual variation. In Petromyzon, as in 

 the Teleosts alluded to above, a radiate envelope is found inside a 

 villous one which becomes swollen up and sticky on the absorption 



