150 PKINCIPLES OF EMBRYOLOGY 



In recent times, Pasteels is the author who has studied in most detail 

 the relation between the grey crescent and the various plasmatic regions of 

 the egg (see liis review, 195 1). The internal contents of an amphibian egg 

 are fairly fluid and if the egg is turned upside down and held with the 

 vegetative pole uppermost, the heavy, yolky material from that end 

 streams down and comes to lie against the cortex near the animal pole 

 region. The grey crescent is not distinct enough in appearance always to 

 be recognisable with certainty in such eggs, but as we have seen, in later 

 development it gives rise to the blastopore and that structure, at least, is 

 unmistakable. It is found that blastopores always appear at the edge of 

 regions in which masses of yolky cytoplasm are in contact with the cortex 

 (Fig. 9.4). At these blastopore regions invagination takes place and an 

 embryo will eventually develop. Its cephalo-caudal polarity is determined 

 by the gradient in yolk content, the future head end always originally 

 lying nearest to the most concentrated mass of yolky cytoplasm. It is 

 clear from these experiments that the position of the grey crescent is 

 determined by the mutual relations of the yolky cytoplasm and the 

 cortex. 



In eggs which have been held upside down and in which drastic altera- 

 tions of the internal structure have occurred, it is difficult to be certain 

 of the nature of the interaction which takes place between the yolky 

 cytoplasm and the cortex. In the normal egg Ancel and Vintemberger 

 have shown that the formation of the grey crescent involves a movement 

 of the cortical layer of that region towards the animal pole (Fig, 9.3B). 

 This seems to carry up with it some of the underlying yolky cytoplasm 

 which thus becomes thoroughly mingled with the marginal cytoplasm 

 with its larger content of basophylic granules and mitochondria. It seems 

 probable that it is this mingling of yolky and marginal cytoplasm which 

 is the essential feature of the grey crescent. If shortly after fertilisation 

 and before the appearance of tlie normal grey crescent an egg is tilted 

 slightly so that the animal-vegetative axis makes an angle with the vertical, 

 the yolky cytoplasm slides down into the lowest position and in doing so 

 leaves behind it a sub-cortical layer which has much the same appearance 

 as that of a normal grey crescent : and as we have seen it eventually devel- 

 ops into a blastopore. This experiment was one of the earliest to be carried 

 out in amphibian experimental embryology. It was originally performed 

 by Born in die 'eighties of the last century. It has recently been studied 

 in detail by Pasteels (195 1). Pasteels also shows what happens when the 

 experiment is carried out slightly later, after the appearance of the grey 

 crescent. In eggs rotated at this time the blastopore will, again, eventually 

 appear somewhere at the edge of the main mass of yolky cytoplasm, and 



