THE TOPOGENESIS OF THE EMBRYO 103 



way, even though invagination has been suppressed, and turned 

 into an evagination. Notochord and somites develop from the 

 chordamesoderm, although the latter is not surrounded by- 

 ectoderm. There is a beginning of differentiation into head, 

 trunk, and tail, and even the first primordia of branchial pouches 

 are formed in the endoderm of the head-region. This shows that 

 the various topogenetic processes are more or less mutually 

 independent. It is essential for normal gastrulation, however, 

 that these processes are linked together in an orderly fashion. 

 Schechtman (1942) has demonstrated that normal rolling in of 

 prospective notochord material, and closure of the blastopore, 

 will only occur if the connections between the parts of the 

 marginal zone are uninterrupted. 



Much of our insight into the nature of the moving forces 

 in gastrulation is due to experiments by Holtf refer (1943-44). 

 He found that the expansive tendencies of the animal field 

 depend on the presence of a tough, semi-elastic surface "coat" 

 which unites all cells into one mechanical unit, and governs 

 their mass-movement. This "coat" shows a tendency to expand, 

 and for that reason the region tends to occupy the largest 

 possible area. The formation of this surface layer is probably 

 due to the so-called Gibbs phenomenon, i.e. accumulation at 

 phase boundaries of substances that lower the interfacial 

 tension. In so far as it is provided with a similar layer, the 

 vegetative field also shows expansive tendencies, but to a lesser 

 extent than the animal field. 



Certain specific attractions and repulsions existing between 

 different types of cells also result in topogenetic processes. 

 Holtfreter (1939), who studied these phenomena, has sum- 

 marized them under the term "tissue affinity''. These forces 

 manifested themselves clearly in experiments in which different 

 tissue elements were cultivated together. Holtfreter found that 

 under their influence parts of a cell mass may become separated, 

 or, conversely, cell groups of different types may fuse and 

 interpenetrate, so that they come into close contact. Such 

 processes play a very important role, particularly in later devel- 

 opment when the organs of the embryo begin to differentiate. 



The following investigations have thrown some light on the 



