vni EGG-ENVELOPES 457 



of water. In the Myxinoids the egg is contained in a character- 

 istic elongated shell provided at each pole with a group of stiff 

 anchoring filaments ending each in a lobed umbrella-shaped expan- 

 sion (Fig. 206). The piece of shell covering the germinal pole is 

 marked off by a deep incision from the rest so as to form a lid which 

 is forced off at the time of hatching. 



Whatever the true nature of the envelopes under discussion, 

 whether primary or secondary, they already exist round the egg 

 before fertilization takes place, and as the substance of the envelope 

 is, as a rule, impenetrable by spermatozoa there necessarily exist one 

 or more openings or micropyles through which the fertilizing sper- 

 matozoon makes its way into the egg. In the Myxinoids one such 

 micropyle is found in the middle of the lid, surrounded by the con- 

 centric circles of anchoring filaments. The presence of a micropyle 

 in Lampreys and in Lung-fishes is not definitely established. In 

 Zepidosiren it has been observed that the envelope enclosing the 

 coelomic, and therefore unfertilized, egg is thick and gelatinous while 

 after fertilization it becomes dense and horny. Possibly therefore 

 during the first-mentioned condition it is penetrable by the sper- 

 matozoa. In Teleostean fishes a micropyle occurs at the apical pole, 

 and so also with Actinopterygian Ganoids except that in the 

 Sturgeons there exist a group of openings (5-13 in the Sterlet, 

 according to Salensky) instead of a single one. 



Of the more complicated arrangements of tertiary envelopes 

 found in Vertebrates no better example could be taken than those 

 found in the case of the Fowl's egg. These will be found described 

 in Chap. X. In Birds in general the envelopes resemble those of the 

 Fowl, differences occurring in details of relative size, shape, and colour 

 of the shell. The " egg " (i.e. the zygote with its envelopes) appears 

 to be largest relatively in Apteryx where it reaches about a quarter of 

 the weight of the parent. 



The shape of the shell is impressed upon it by the pressure of the 

 oviducal wall and differences in shape are no doubt due to differences 

 in the peristaltic contraction. Thus the strong contraction of the 

 oviducal muscles which, acting on the headward side of the egg, 

 propels it onward, if combined with comparatively slight contraction 

 on the tailward side of the egg will naturally cause the egg to assume 

 a more or less markedly conical shape — the end of the egg directed 

 towards the cloaca being broader than the other end. In some 

 cases, as that of eggs laid on bare ledges of rock, this conical 

 shape has probably had a definite natural selection value, in caus- 

 ing any rolling movement of the egg to follow a strongly curved 

 path. In other cases where there is less marked inequality of 

 pressure on the two poles of the egg the shape is more regularly 

 ellipsoidal. 



The eggs of Birds being commonly exposed to light and to view 

 they very often show a characteristic colouring, either throughout 

 the thickness or merely in the outer layer of the^ shell. In very 



