SECT. 3] AND ORGANISATION 577 



becoming the stimulated. The optic vesicle may then be termed 

 from this point of view an organiser of the second grade". The same 

 state of affairs exists between the medullary plate and the roof of 

 the archenteron. Spemann showed that a piece of presumptive 

 ectoderm, e.g. a piece of presumptive medullary plate, could be 

 made to develop into archenteron when implanted into the dorsal 

 lip of the blastopore and allowed to pass inwards with the gastrular 

 invagination. But, on the other hand, the same experiment can 

 be reversed, and a piece from the roof of the archenteron taken, 

 grafted somewhere else and so allowed to induce a medullary plate 

 in any indifferent epidermis. "Geinitz", said Spemann, "recently 

 combined these two experiments into one. A piece of presumptive 

 epidermis was removed at the beginning of gastrulation from an 

 embryo of Triton taeniatus, heavily stained with intra-vital stains and 

 implanted into the dorsal lip of the blastopore of another unstained 

 embryo at the same stage of development, in such a manner that 

 it invaginated and formed a portion of the archenteron. It was then 

 removed again and this time transplanted into the cleavage cavity 

 of a third embryo at the onset of gastrulation. In the course of 

 gastrulation it came to lie under the ectoderm and induced in the 

 latter the formation of a secondary medullary plate. This transplant, 

 if left in its normal environment, would have differentiated into 

 epidermis ; under the influence of the archenteron — ^into medullary 

 plate, perhaps into eye-anlagen and would then have induced the 

 formation of a lens. In its third environment it became itself an 

 organiser and induced the formation of a secondary medullary plate." 



What is the extent of the region occupied by the organiser? This 

 question has been investigated by Bautzmann, who definitely settled 

 it by transplanting small trial pieces from the whole surroundings 

 into the blastocoele cavity of other gastrulae. Gastrulation brings 

 these under the ectoderm and then their power of induction, if they 

 have any, will manifest itself, as Marx showed, by the appearance 

 of a medullary plate in the overlying ectoderm. The region of the 

 gastrula, then, which may be said to contain the organiser, is a semi- 

 circular area above and beside the upper lip of the blastopore. 

 Normally it is invaginated, and gives rise to notochord and mesoderm. 



A good deal is known about the structure of the organiser region. 

 It must have an axial longitudinal structure which is not lost by 

 transplantation, for the second embryo may readily be obtained 



NEI 37 



