REGENERATION IN EGG AND EMBRYO 



That the uninjured blastomere may at first segment as a half is not 

 improbable, but that whole embryos are formed only by the formation 

 of new material at the side of a half-embryo is, I think, hardly possible, 

 since the results of Schultze, Wetzel, Hertwig, and myself show that 

 a whole embryo may develop directly out of the material of a single 

 blastomere. 



Spemann (1900) has carried out some novel experiments on the 

 eggs of triton, and has shown how in another way double structures 

 may be produced. If a ligature is tied loosely around the egg at 

 the first cleavage exactly along the division plane between the first 

 two blastomeres, it will be found later that the long axis of the single 

 embryo lies, in the great majority of cases, across the ligature, and 

 only in a small percentage of cases does the median plane correspond 

 with that of the ligature, and, therefore, with the first cleavage plane. 



If one of the latter eggs is allowed to develop to the blastula 

 stage, and the ligature is then drawn tighter, so that the blastula 

 is completely constricted, an embryo develops from each half. 



If one of the former eggs is allowed to develop to a stage when the 

 medullary plate is laid down, but is not yet sharply marked off, and 

 the ligature is then tightened, there will be formed (the plane of con- 

 striction being across the medullary plate) from the anterior part a 

 normal head with eyes, nasal pits, ears, and a piece of the notochord, 

 and from the posterior part there will be formed, at its anterior end, 

 another new head just behind the ligature. Ear-vesicles develop in 

 this part at the typical distance from the anterior end. The brain 

 that develops has a typical cervical curvature, and eye evaginations 

 appear at the anterior end. The chorda, that extended at first to the 

 anterior end of this region, is partially absorbed. 



If the ligature is drawn tighter at a later stage, when, for instance, 

 the medullary plate is plainly visible but is still wide open, a different 

 result is obtained. The posterior part no longer forms a new head at 

 its anterior end, but develops into those structures that it would form 

 normally. In some cases it was found that the region from which 

 the ear develops had been pinched in two, and in consequence a 

 small vesicle appears in front of the constriction and another behind it. 



In those cases in which the ligature lies in the median plane of the 

 embryo, it is found that a double anterior end is produced. As the 

 embryo develops it tends to elongate, and in consequence the mate- 

 rial is pushed forward on each side of the ligature. A double head is 

 the result. The extent of the doubling depends on the depth of the 

 constriction between the halves. In the most extreme cases two com- 

 plete heads are formed with an inner nasal pit, eye, and ear on each 

 head, as well as the normal outer ones. The results show that even 

 such complicated structures as the eyes and ears, etc., may arise 



