THE DEVELOPMENT OF ORGANISMS 751 



This mesoderm becomes divided into an inner layer, which clings 

 to the gut, forming its muscular sheath, and an outer layer that 

 clings to the skin, forming the dermis. The cavity between these 

 two layers is the body-cavity. 



The folding off of the notochord, and that of the mesoderm, 

 alike proceed from the anterior region backwards; and they merge 

 posteriorly in the rim of the blastopore, where there is a zone of 

 actively growing cells, which contribute to all the three germinal 

 layers — ectoderm, mesoderm, and endoderm. This activity on the 

 part of the blastopore-lip tends to the elongation of the embryo; 

 and this activity continues even after the blastopore lips draw 

 together into a narrow slit and then close altogether. 



Meanwhile changes are proceeding along the middle-dorsal line 

 of the ectoderm. A neural plate becomes by unequal growth a neural 

 groove, and the neural folds bounding this groove bend towards one 

 another, to form the walls of the neural canal, of which the anterior 

 part widens into the brain; while posteriorly the two neural folds 

 grow around the blastopore. 



All this preamble, which will become clear if good figures are 

 carefully studied, is necessary to an understanding of one of the 

 most interesting results of Spemann's experiments. In early em- 

 bryonic stages, before gastrulation, there is great plasticity. It is 

 not irrevocably settled what a particular patch of cells will form, if 

 its position in the embryo is altered. In other words, differentiation 

 is not yet irreversible. 



But in the few hours of gastrula-formation this early plasticity 

 disappears. By a sudden change "the germ loses its capacity for 

 regulation, and becomes a sort of jig-saw of separate parts, each 

 predetermined to produce just one particular organ of the future 

 tadpole". This sudden change has been traced by Spemann to the 

 influence of the active cells of the dorsal lip region. 



Spemann showed that if the minute dorsal lip-region of the frog 

 or newt embryo be grafted upon, say, the flank region of another 

 embryo, it there induces the development of an extra set of dorsal 

 organs — such as neural canal and notochord, and of other structures 

 as well, even to muscle segments and kidney-tubes. In fact, at first 

 sight it looks as if this graft were starting a second embryo. But 

 these new structures are not formed //'om the grafted fragment, but 

 arise in the tissues (thus abnormally stimulated) in contact with 

 it, and which were in course of their normal and quite different 

 development. By reason of this strange and potent influence of this 

 particular grafted portion of one embryo upon some independent 

 portion of another, Spemann called the former an "organisator". 



Two points should be emphasised: (i) that in the frog embryo 

 no region but the dorsal lip-area has this organising power; and 

 (2) that it has this power when it is the most active part of the 



