148 PRINCIPLES OF EMBRYOLOGY 



since as the egg cleaves up into many cells, the even colour of the animal 

 pole becomes broken up by the cell boundaries so that it merges into the 

 paler tint of the crescent, v^hich ceases to be recognisable. But we can 

 apply the technique of vital stained marks (p. 158). A small spot of colour 

 placed in the centre of the grey crescent is found eventually to lie in the 

 dorsal midline of the animal, and, in fact, somewhere in its notochord. 

 The grey crescent therefore marks the dorsal side, and, in its position, 

 corresponds to the grey chorda-neural crescent which appears, at a 

 somewhat later stage, in the ascidian egg. 



The appearance of the grey crescent and the marking of the dorsal side 

 is the first great step in embryonic development in the Amphibia. Natur- 

 ally it is important to know its causal antecedents and its causal conse- 

 quences; what brings it about, and what effect does it have? There is 

 general agreement about the latter. After the grey crescent has appeared, 

 every developmental performance of the egg is related to it. If, for in- 

 stance, the egg is cut in half (which can be done even before the first 

 cleavage by putting the egg into a loop of fme hair which is slowly pulled 

 tight (Fig. 9.2), then only those halves which contain some grey crescent 

 material will develop any of the main embryonic tissues, such as neural 

 system, notochord, somites, kidney, etc.; ventral portions of the egg 

 which have no crescent material, usually form only skin and disorgan- 

 ised mesoderm and endoderm not recognisable as any definite tissue, 

 though Dollander (1950) has recently shov^oi that in some cases a certain 

 degree of regulation occurs and the ventral parts also produce a httle 

 neural tissue and axial mesoderm. The grey crescent, in fact, is the pre- 

 cursor of the 'organisation centre' of the gastrula, which, as we shall see, 

 (p. 175) is the agent which causes the formation of the rest of the embryo. 



The antecedents of the grey crescent are less well understood (Ancel and 

 Vintemberger 1948, Pasteels 195 1). Its position is certainly not completely 

 fixed before fertilisation, since it is possible by suitable treatment to make 

 it appear in any desired meridian of the unfertilised egg. If, for instance, 

 an egg is fertilised with sperm brought on a needle to a given place on the 

 surface, the grey crescent usually appears at or near the diametrically 

 opposite side. Similarly, if the newly fertilised egg is held for some time 

 so that one side of the yolky hemisphere is much higher than the other, 

 the grey crescent usually appears on the higher side. 



The experiments of the last paragraph demonstrate that the grey cres- 

 cent is not fixed before fertihsation, or indeed much before it actually 

 appears. But the same experiments also suggest that there is a pre-disposi- 

 tion of a certain plane to become the plane of symmetry. The experiments 

 of tilting the egg, or of fertilising it in a definite place, do not always 



