98 CLEAVAGE AND DIFFERENTIATION 



half. The same mosaic development is seen in anterior and 

 posterior 2/4 halves (fig. 45). 



In the early period of study of experimental embryology, these 

 two types were sharply distinguished from one another as "regula- 

 tion-eggs " and "mosaic-eggs " respectively. Later work has, how- 

 ever, shown first, that all forms do not fall into one or the other of 

 two sharply marked categories, but that the two extremes are con- 

 nected by a complete series of intermediate steps ; and secondly, that 

 at least two very distinct processes impeding complete regulation 

 maybe operative in "mosaic-eggs" (pp. 105, 108). Furthermore, it 

 appears that all developing organisms at some stage of their career 

 possess the power of regulation, but lose it at some later stage. Thus 

 the distinction between " regulation-eggs " and " mosaic-eggs " loses 

 a great deal of its theoretical importance, and if the terms are to 

 continue being used, it is best that they should be employed in a 

 purely descriptive sense with reference to their behaviour during 

 cleavage. 



The most extreme case of regulation is that of the Hydrozoa, 

 already cited, in which single blastomeres from either the animal 

 or the vegetative regions of the egg will develop into larvae as if 

 they were whole eggs. But in a number of forms, the diflFerentiation 

 along the main axis of polarity of the egg is sufficiently fixed by the 

 time of fertilisation to render this impossible, while differentiation 

 round the main axis is still absent or so slight as to permit of regu- 

 lation in a fragment containing all levels of the egg along its main 

 axis. 



In most eggs, latitudinal division does not occur until the third 

 cleavage (leading from the 4- to the 8-cell stage), and this means 

 that isolated 1/8 blastomeres, or isolated animal or vegetative 

 halves, will be unable to give rise to whole larvae, whereas 1/2 or 

 1/4 blastomeres, or isolated lateral halves, will be capable of com- 

 plete regulation. This is the case, for instance, in Echinoderms^ 

 and to a certain extent Nemertines'^ (fig. 44). 



The eggs of Amphibia approach this last type, but the capacity 

 of their blastomeres to achieve complete development is limited 

 by the restriction of organiser capacity to the dorsal side. The 

 organiser-region is determined at fertihsation, and therefore 1/2 or 



1 Driesch, 1900. ^ E. B. Wilson, 1903; Zeleny, 1904. 



