REARRANGEMENT OF PRESUMPTIVE STRUCTURES DURING GASTRULATION 65 



1, just above the lip of the blastopore, is now located internally in an anterior 

 position, forming, as we have shown by vital staining, the head endoderm. 

 Regions 2, 3, 4, and 5 stretch backward in a posterior direction. Region A, 

 just below the dorsal lip, is now covered over and is the most anterior region 

 of the floor of the archenteron. And regions B, C, D, E, and F stretch 

 posteriorly. 



It is quite obvious that what has happened in a general way is that 

 regions 1 and A have become located internally as a result of both a folding 

 process and an overgrowth of cells. The blastocoele has been obliterated, its 

 cavity becoming smaller and smaller. And as 1 and A become located 

 internally, 2 and B converge; they become pulled in. Regions 3 and C like- 

 wise converge and take an internal position. The whole process is one of 

 bringing the external surfaces into a cavity forming the roof and floor of 

 the archenteron. 



At the same time it might be well to point out the structures which 

 regions 1 through 5 represent. Region 1 represents head endoderm, as can 

 be seen from the map. Regions 2 through 5 represent notochord; this means 

 that the region just above the dorsal lip becomes located anteriorly. That is 

 to say, 1 is the anterior end of the archenteron in the late gastrula; 5 is the 

 posterior end in the late gastrula. A through F form the floor of the archen- 

 teron, A being the most anterior part, F being the most posterior. These 

 regions which become located internally originally comprised about half the 

 external surface of the egg. Now the rest of the external surface of the egg 

 must be stretched to replace these parts which have become internal. 



A glance at the map, shows that the region of the presumptive neural 

 plate must be stretched out into a plate just above the roof of the archenteron 

 during the process of gastrulation. The remainder, the presumptive epidermis, 

 must be stretched from the extreme anterior end all the way around to the 

 posterior end to cover the surface of the embryo. This stretching is actually 

 accomplished by a rearrangement of the cells in the ectoderm. The ectoderm 

 is very thick in the early gastrula and may be about six cells in thickness. 

 At the end of gastrulation the ectoderm is much thinner — perhaps only two 

 cells thick. In changing from six to two cells in thickness the surface of the 

 ectoderm would be increased three times. 



The mechanics of gastrulation are rather complicated. They involve a 

 migration of the lips of the blastopore over the yolk (epiboly) and a turning 

 in of cells over the lips of the blastopore (involution). These processes can 



