242 PRINCIPLES OF EMBRYOLOGY 



exhibit a most remarkable range of conditions, most of which can be 

 considered as twinnings or duphcations of various kinds, ranging from 

 almost complete and separate twins to doublings of the main axis, of the 

 heart, the allantois, etc. There are also many examples of single more or 

 less complete embryos accompanied by ball-shaped lumps of unorganised 

 tissue. Gluecksohn-Schoenheimer compares the structures found to those 

 which result from the partial or complete separation of the first two 

 blastomeres in Amphibia. If this comparison is accepted, and it seems quite 

 convincing, one would have to conclude, firstly that the mammal egg 

 at an early stage is capable of profound regulation (a point already clear 

 from the occurrence of identical twins and directly confirmed by the 

 evidence from blastomere separation mentioned above [p. 238]); and 

 secondly that it possesses an organisation centre which is, or becomes, 

 locahsed as does the grey crescent in the Amphibia or the posterior part 

 of the endoderm in birds. The most plausible explanation of the action of 

 Kink is that it interferes with the gradual regionalisation within the egg, 

 by which the organisation centre becomes focused on to the dorsal side. 

 Unfortunately the evidence does not help us to decide at what stage in 

 development this takes place in mammals. In the armadillo, in which four 

 identical twins are normally produced from each egg, the initiation of 

 the four rudiments appears to occur rather late, in the blastocyst stage 

 (Patterson 191 3), and although one would now like to see the newly 

 fertihsed eggs of this form re-investigated by the methods of Dalcq, it is 

 perhaps likely that the mammals are like teleosts in that their dorsal plane 

 is not finally fixed till a relatively late stage. 



It may be mentioned that all the mouse genes just mentioned (T, t°, t^ 

 and Kink) are closely linked in the same chromosome; they may all be 

 chromosome aberrations rather than true point mutations. Many other 

 genes with generally similar effects (mostly not yet analysed embryologic- 

 ally) are known in the same chromosomal region. The causes for the 

 association of this part of the chromosome with the primary organiser 

 phenomena is quite unknown and present a very intriguing problem. 

 Gluecksohn-Schoenheimer very tentatively suggests that possibly the 

 spatial pattern of the genes in the nucleus may have some connection with 

 the formation of the patterns of the early embryonic fields. 



5. Comparative geography of the presumptive areas 



In the past, attempts to compare the processes of gastrulation in the 

 different classes of vertebrates have been made in terms of concepts which 

 were derived from the static shapes which particular embryos may assume 

 at certain points in their development. For instance, one tried to 



