THE TOPOGENESIS OF THE EMBRYO 99 



under the notochord, and fuse so that the archenteron is now 

 surrounded by endoderm on all sides. From then on it becomes 

 the gut of the embryo (Fig. 34 h). 



In this way, the processes of gastrulation and neurulation, 

 which follow each other without delay, transform the simple 

 structure of the blastula into the complicated architecture of 

 the embryo, in which the primordia of several organ systems 

 can already be recognized. 



Only a small part of the movements here described can be 

 seen by direct observation of the germ. The development and 

 closing of the neural plate can be easily seen, but the shifting 

 processes during gastrulation, as in the infolding of marginal 

 zone material, can be made visible only by means of special 

 techniques. Our insight into these processes has been greatly 

 deepened by the method of colour marking intra vitam, invented 

 by Walter Vogt (1925-29). In this method, small marks are 

 made on the surface of the germ with so-called "vital" stains, 

 i.e. dyes which do no damage to living cells. In this way, we 

 can investigate the particular organs of the embryo into which 

 the cells of the various areas of the germ will eventually find 

 their way, and these organs of the embryo can, so to speak, be 

 projected back onto the surface of the blastula (Fig. 35). 



All movements involved in gastrulation and neurulation are 

 due to active changes in shape of the cells of which the germ 

 consists. Motility is one of the elementary properties of all 

 cells. Most cells are capable of active changes in shape, although 

 not all cells show this so clearly as the elements of the 

 musculature. Many animal eggs, too, are motile to a lesser or 

 greater extent. In some lower animals, such as the sponges, 

 the eggs can crawl around freely in the maternal tissues. These 

 eggs are very similar to amoebae. In other animals, the eggs 

 show amoeboid movements only at certain times, e.g. in the 

 snail Limnaea immediately after the expulsion of the polar 

 bodies. We can imagine that this power of locomotion becomes 

 specialized in divergent directions in the various cell groups 

 of the germ, due to the modifications in the physico-chemical 

 condition of the parts of the egg, caused by chemodifferentia- 

 tion. The cells of one area will show a tendency to expand 



