i68 organisers: inducers of differentiation 



however, developed in animal halves which have been isolated 

 later than this. This result can only be understood on the view 

 that the vegetative half of the egg and embryo contains a factor 

 whose presence and action for a certain minimum period of time 

 is essential for the production of a stomodaeum in the animal 

 half.i 



Further experiments suggest that this factor is situated at the 

 vegetative pole of the egg, where invagination takes place and the 

 blastopore arises. Recent improvements in technique have made 

 it possible to assemble certain definite blastomeres, or groups of 

 blastomeres, of the sea-urchin, at will. At the i6-cell stage, there 

 are normally eight mesomeres (presumptive ectoderm) : four macro- 

 meres (the animal half of each of which is presumptive ectoderm, 

 the vegetative half, endoderm) : and four micromeres (presumptive 

 mesenchyme). Embryos artificially assembled and consisting of 

 sixteen mesomeres, four macromeres, and four micromeres; or of 

 the even more abnormal combination of twelve mesomeres, two 

 macromeres, and two micromeres (in each case, therefore, contain- 

 ing too much presumptive ectoderm), develop into normal pluteus 

 larvae. There is present, therefore, a regulating agent which organ- 

 ises the available material to form a harmoniously proportioned 

 larva. That this agent is situated at the vegetative pole of the egg is 

 probable from the facts that vegetative tissue must be present if 

 gastrulation is to take place at all, and that the micromeres (which 

 occupy the vegetative pole) are predetermined to initiate invagina- 

 tion, and do so wherever they may be grafted. Further, if an embryo 

 at the i6-cell stage is divided meridionally and the two halves are 

 stuck together again so that their axes of polarity are reversed in 

 respect of one another, invagination takes place at each end, where 

 the micromeres are situated, and the resulting larva is a double 

 monster, with two guts, skeletons, etc. This can be understood if 

 the micromeres act as organisers^ (fig. 83). 



But, as in the case of Cory?norpha, this sea-urchin organiser is not 

 specifically located in or restricted to the micromeres, for if they 

 are removed, the next most vegetative material can function as an 

 organiser, and induce the formation of a fairly normal pluteus larva. 

 But if no material from the vegetative hemisphere is present, there 



^ Horstadius, 1928. 



