146 PRINCIPLES OF EMBRYOLOGY 



water; normally the eggs are fertilised either as they leave the female's 

 body (in frogs) or by sperm which the female has taken up into her cloaca 

 (in newts). The egg within the ovary and oviduct is surrounded by a 

 viscid layer of jelly, which swells on contact with water into a thick 

 protective covering; in newts each egg is enclosed within a separate 

 capsule, but in frogs a whole clutch of eggs coheres into a single mass 

 within which the individual eggs are scattered. 



Amphibian eggs contain moderate quantities of yolk; much more than 

 non-yolky types such as echinoderms or ascidians, much less than the 

 extremely yolky birds' eggs. The yolk is present in the form of small 

 platelets or ovoid granules, and, although scattered throughout the whole 

 egg, is more concentrated at the vegetative end where the granules are 

 also larger in size. Near the opposite, animal, pole is a large germinal 

 vesicle fdled with clear sap. The heavy load of yolk granules makes it 

 rather difficult to distinguish different regions of cytoplasm, such as those 

 so well shovel in the ascidians, and it is only recently that the earliest 

 phases of development are becoming clear (Reviews: Pasteels 195 1 ; Dalcq 

 1950&). There is, for instance, always a peripheral zone or cortex which 

 contains little yolk; the animal half of it usually carries a considerable 

 number of pigment granules, which make up a dark animal cap which is 

 very clear in the frog; the outermost layer of all is a relatively impervious, 

 very extensible membrane, the 'coat' (Holtfreter 1943a). hi the interior of 

 the egg, it is usually possible after the germinal vesicle has broken down 

 to distinguish a clearer animal plasm, in which the yolk granules are 

 smaller and more scattered; this appearance may be due to the admixture 

 of the nuclear sap (Fig. 9.1). In some forms (the frog and axolotl) 

 there is also a darker 'marginal plasm' which lies in a circle fairly close 

 under the surface j ust above the equator, and in the frog there is another, 

 central, region with small yolk platelets (Pasteels 1951). 



The pattern of cytoplasmic regions in the unfertihsed egg is thus radially 

 symmetrical and shows no obvious sign of the future dorso-ventral plane 

 of symmetry. An indication of this plane does, however, appear fairly 

 soon after fertilisation in many amphibian eggs. The first apparent result 

 of fertilisation is a slight lifting-off of the inner or vitelline membrane 

 from the egg, which thus becomes free to revolve within its jelly capsule 

 and lie in its natural position with the heavy yolk-laden vegetative pole 

 downwards; the swing round into this position takes only a few minutes. 

 Soon afterwards, in the frog and some other species, a 'grey crescent' 

 appears on one side, lying between the dark animal hemisphere and the 

 pale yolky vegetative end. This is destined to play a fundamental part in 

 later development. Its fate cannot be followed without special methods. 



