172 PRINCIPLES OF EMBRYOLOGY 



shown. For instance if two gastrulae are cut in half transversely and the 

 two halves containing the blastopores combined, the two forward-moving 

 streams of axial mesoderm meet one another before they have completed 

 their elongation. As Spemann showed (cf. 1938) they then combine and 

 spread out to the two sides, so that finally double embryonic axes are 

 formed in a cross-shape (a so-called Duplicitas cruciata). Again, if a fragment 

 is taken from a region where the movement is not very intense (e.g. the 

 lateral or ventral mesoderm in the newt, or the more posterior parts of 

 the primitive streak in the chick) and is grafted near a region of active 

 movement (such as the dorsal lip of the blastopore or the anterior primitive 

 streak), the inherent tendencies of the graft often appear to be swamped by 

 the more powerful tendencies of its new surroundings. And this does not 

 seem to be merely a question of the graft being physically swept along 

 in the tissue streams of its new location, but the phenomena suggest that 

 the graft is as it were infected with the characteristics of its neighbour- 

 hood (Spemann and Geinitz 1927). 



This infection of a graft with the dynamic tendencies of its surroundings 

 has an important bearing on the classical problem of 'the specificity of the 

 germ-layers'. The older embryologists, relying entirely on descriptive 

 methods of analysis, tended to reach the conclusion that ectoderm, meso- 

 derm and endoderm were three fundamentally distinct types of tissue 

 from a combination of which the embryo was built up. Soon after 

 methods of experimental attack were discovered in Amphibia, however. 

 Mangold (1925) showed that pieces of prospective ectoderm, if grafted 

 into the mesoderm region in front of the blastopore, became invaginated 

 with their surroundings, and thereafter behaved in every way as meso- 

 derm, and also that ectoderm could be converted to endoderm in a 

 similar way. In birds the demonstration is less complete, but Waddington 

 and Taylor (1937) found that pieces of prospective ectoderm grafted into 

 the primitive streak could become mesoderm, provided they became 

 well enough assimilated to their surroundings for the coherent tissue to 

 break down into single cells which migrate separately into the middle 

 layer. These experiments show that there is no profound and permanent 

 physiological difference between the three layers. 



SUGGESTED READING 



Vogt 1929 is a classical paper (in German, but the pictures should be studied). The early 

 development of Amphibia is described in many texts (e.g. Nelsen 1953, Spemann 1928). 

 Lehmann 1945, Fankhauser 1948, Dalcq igsob, Pasteels, 1951 add important information 

 on early stages. For the chick, Waddington I952(j, Chapter 2, Hamilton 1953, Rudnick 

 1948. 



