502 T. GUSTAFSON 



time-space pattern. As an example we may study the release of the 

 pulsatory activity. This activity 



starts in zone i and brings about a release of the primary mesen- 

 chyme cells into the blastocoel; 



continues in zone 2 where it brings about the primary invagination 

 of the archenteron rudiment ; 



continues in zone 3 where it brings about an early evagination of 

 the coelomic sacs ; 



continues in zone 4 where it may contribute to the morphogenesis 

 (a rounding up) of the oesophagus; 



continues (occasionally) in zone 4. 



Fig. 4. The bending of the archenteron towards the presumptne stumodaeum. 

 a shows how contact has been formed between the archenteron tip and the ventral 

 side by means of pseudopods. The tension in the pseudopods has caused the 

 formation of cones of attachment in the ectoderm and pulled out cells from the 

 archenteron tip, the whole of which will gradually (through the pull of several 

 pseudopods of this kind) be forced to make contact with the ventral side. In b the 

 contact has been established. The archenteron tip is sometimes met by the 

 invaginating stomodaeum rudiment, an invagination which recalls the primary 

 invagination of the archenteron. 



The pulsatory activity in the other zones generally fails, with some 

 exceptions. It may thus occur (preliminary studies) in the ectoderm where 

 it mav contribute to the invagination of the presumptive stomodaeum [2]. 

 It also occurs in the regions between the extending arms and thus con- 

 tributes to their elongation [4]. 



The difference in the morphogenetic activity of the consecutive zones 

 is not onlv a time difference. There also appears to be a decrease in the 

 capability of the cells to pulsate and to emit pseudopods and an increase 



