THE GERM-LAYERS IN REGENERATION 211 



atrial cavity. In all these forms the inner tube gives rise to the new 

 pharyngeal cavity of the bud, while this same cavity comes from the 

 endoderm of the archenteron of the embryo. In the bud embryo the 

 peribranchial space is also derived from the inner tube ; hence it is 

 endodermal in the first series, and ectodermal in botryllus. In the 

 egg embryo it is ectodermal. In regard to the development of 

 the nervous system there is some difference of opinion. A number 

 of investigators have found that the new brain arises from the outer 

 part of the inner or branchial tube, which has in most cases an 

 endodermal origin. Seeliger and Lefevre believe the nervous sys- 

 tem to arise from mesodermal cells that lie between the two tubes. 

 It appears, nevertheless, that in several forms the brain really comes 

 from the inner tube, which also gives rise to the branchial sac. There- 

 fore, in those cases in which the inner tube is endodermal the brain 

 has the same origin, and in the case in which the inner tube is ecto- 

 dermal, the brain is ectodermal, but the pharyngeal sac has also an 

 ectodermal origin. There is obviously no definite relation between 

 the origin of these structures in the bud and in the egg embryo. 



A similar difficulty is met with in the Bryozoa in regard to the 

 development of the egg embryo and the bud embryo. 



Braem, who has made a critical examination of the germ-layer 

 theory, 1 has found it impossible to give a morphological definition of 

 a germ-layer, and has adopted a physiological criterion. He thinks 

 that in whatever way a germ-layer arises, whether by folding, or by 

 delamination, etc., it exists independently of its method or place of 

 origin. A layer is not endodermal because it forms the inner wall 

 of a gastrula, but it is endodermal because it develops into the diges- 

 tive tract. The germ-layers of different forms are only similarly 

 placed, but whether they are homologous will depend on other 

 things. On this view the inner tube of the ascidian bud that gives 

 rise to both digestive tract and to the nervous system is simply an 

 indifferent layer until it gives rise to these structures. Its cells 

 may be looked upon as indifferent, as are those of the blastula. 

 Thus the difficulty of the morphologist is not solved, but the knot is 

 cut. For Braem the germ-layers are convenient terms, since he 

 rejects any historical significance that they may have, and it is just 

 this side of the question that the morphologist has attempted to work 

 out. While the evidence shows that the germ-layers cannot have 

 any such final attributes as embryologists have attempted to assign 

 to them, and that Braem has called attention to the real and impor- 

 tant problems connected with the study of development, yet it may 

 still be admitted without endangering the newer point of view, that 

 there may be also an historical question in connection with the germ- 



1 Biologisches CentralMatt, XV, '95. 



