448 



PRINCIPLES OF EMBRYOLOGY 



Whatever the detailed chemical mechanism, it seems legitimate to think 

 of the membranes of cells which are in contact with one another as be- 

 having as though they were being forced together, or forced apart, by a 

 number of zipp-fasteners, each with a characteristic tension tending to 

 extend the zipped region. Such a system would behave very like a group 

 of cells with truly fluid membranes possessing true surface tensions. 



It is very probable that forces of a similar kind, arising from the 

 adhesiveness of the cell membranes, play a part not only in the in-turning 

 of the invagination stream but also in producing it. It is certain that 

 during the elongation and narrowing of the dorsal material, and the 

 compensating expansion of the more ventral parts of the gastrula, there 

 are considerable changes in the mutual contacts between the cells. One 

 evidence of this is the fact that the layer of tissue forming the blastula 

 roof, which is several cells thick at the beginning of gastrulation, is re- 

 duced to a thickness of little more than two cells by the time gastrulation 



Figure 20.18 



Four successive stages in gastrulation in Tritums alpestris. A group of four, 

 and a group of three, cells have been followed, to show the changes in 

 arrangement of the cells accompanying the streaming towards the blasto- 

 pore tip. (Original, from a time lapse cine film.) 



is completed. Again, if cells are watched approaching the blastopore lip 

 it will be found that they have slid in between one another in such a 

 fashion that a group originally arranged in Hnes lying parallel to the 

 blastopore lip has become rearranged into a longer and narrower shape 

 (Fig. 20.18). A similar interdigitating movement of cells continues on the 

 floor of the neural groove. The elevation and folding together of the 

 sides of the groove may, in fact, be partly caused by the continuous 

 anterior-posterior stretching of its midline, just as the edges of a piece of 

 rubber will roll up on each side of a hne of stretch. 



