290 



histological structure of the balancers developed from transplanted ecto- 

 derm is practically the same as normal. A connective tissue core and 

 a basement membrane are present. 



A number of experiments were made in which the ectoderm of 

 the anläge of the external gills was transplanted upon another embryo. 

 The operations were made when the anläge was in the form of a 

 rounded elevation such as is shown in Fig. 1 (g). In no case did a 

 gill develop from the transplanted tissue. 



Discussion of Results. 



The above results may be recapitulated as follows: 



The ectoderm of the balancer-forming region of a 3 mm embryo 

 has the power to develop a balancer, though of smaller size, when trans- 

 planted without any mesoderm upon the body of another embryo. 

 Under its influence the underlying undifferentiated mesoderm is drawn 

 in to form the core of the balancer. In the development of the 

 balancer therefore the controlling influence is exercised by the ecto- 

 derm. The ectoderm is specialized and the mesoderm seems to be 

 indifferent ^). 



"When some of the underlying mesoderm is transplanted with the 

 ectoderm of the balancer-forming region, the balancer grows larger 

 and more readily. This is possibly due to the mesoderm, but it seems 

 probable that it is due to the ectoderm's being less injured when trans- 

 planted with the underlying tissue. 



The ectoderm of the anläge of the external gills seems to be in- 

 dijfferent. The transplanted ectoderm never shows any indications 

 of the formation of gills. It is at least not specialized to the same 

 extent as the ectoderm which forms the balancers or that which forms 

 the nasal pit. 



The balancer shows no power of regeneration at any stage of 

 development in the direction of its long axis; but the anläge may 

 be partly regenerated if removed before any distinct elevation is 

 present. 



1} A possible source of error suggests itself here. One cannot be 

 positive that absolutely no mesodermic cells are removed with the 

 ectoderm to be transplanted, though it is easy to see when any appre- 

 ciable amount of mesoderm adheres. It is even probable that some of 

 the semifluid ground substance of the mesoderm is transplanted with 

 the ectoderm. It has been assumed in this paper that if any mesodermic 

 tissue at all were transplanted with the ectoderm it was not sufficient 

 in amount to materially affect the result. 



