58o ON INCREASE IN COMPLEXITY [pt. iii 



It is to be noted that all the experiments which have been de- 

 scribed have been carried out on amphibian material, but the 

 evidence which leads us to see in them a validity over all types 

 of embryo, even mammalian ones, is rapidly accumulating. Seidel 

 has extended the concept to insect eggs, while von Ubisch ; Runnstrom ; 

 and Horstadius find evidences of it in those of echinoderms, Wilson 

 in those of annelids, and Graper; Hunt and Waddington in those of 

 birds. It is highly probable that organiser phenomena will in time be 

 found to exist in all varieties of embryo. The division of embryonic 

 development into three main periods, however, is more generally 

 certain, and may be taken to hold in all cases. 



As we have already seen, the junction between the first and second 

 periods of development varies with different eggs. In some (regula- 

 tion eggs), the point of chemodifferentiation does not occur till 

 gastrulation — this is its latest point — but in others it occurs earlier, 

 at some time during cleavage and blastula formation, while in pure 

 mosaic eggs, of which few are known, it occurs before fertilisa- 

 tion. The duration of the second period, the period during which 

 irrevocable differentiation is going on, is rather variable. It ends 

 with the beginnings of function on the part of the foetal organs. 



3-7. Functional Differentiation 



In this third period further differentiation may be dependent 

 on functional activity for its proper progression. The classical 

 example of these mechanisms is the circulatory system, in which 

 Oppel & Roux found that the structure and constitution of blood- 

 vessels depended largely on how they were being utilised by the 

 circulation as a whole. Fischer & Schmieden, for instance, trans- 

 planted a section of vein into the course of an artery, where it acted 

 perfectly well, but took on the characteristics of an artery, i.e. its 

 connective tissue content increased and its muscular walls were more 

 than doubled in thickness. Exactly the same thing happens with 

 regard to the central nervous system, and to the bones. "The 

 formation of the normal structure of the bones ", concluded Landauer, 

 "is caused largely by the static conditions of muscle tonus during 

 embryogeny." Diirken's well-known experiments may be men- 

 tioned, in which, when the hind limb buds of a frog embryo are 

 removed, the hind brain does not develop normally. Again, Babak 

 observed that in tadpoles the area of the active intestinal absorptive 



