413 



Concrescence. 



The fact that the primitive segments extend into the embryonic 

 rim, and are subsequently drawn into the axial embryo, has an im- 

 portant bearing upon the question of the formation of the latter. It 

 serves to strengthen the view that the germ-ring represents, or 

 originally did, the divided halves of the embryo, and that is is formed 

 in part by their opposition. Thus, the doctrine of concrescence receives 

 support from my observations. 



This suggestion will at once bring to mind the recent experiments 

 of Dr. Morgan on Teleost embryos. He shows that the embryo con- 

 tinues to elongate after severing both sides of the germ-ring and 

 assumes that this is fatal to the theory of concrescence. But, it does 

 not seem to me (depending upon his preliminary report in Nos. 23 

 and 24, VIII. Jahrg., of this Journal), that these observations are 

 necessarily opposed to that hypothesis. There are to my mind these 

 two questions involved: 1) how is the embryo formed under normal 

 conditions? and 2) what is its behaviour under materially changed 

 conditions? The answer to the latter does not include the answer to 

 the first. Morgan's experiments undoubtedly show, that it is possible 

 for the constructive material to be brought into the median plane, 

 and for the embryo to elongate, when entirely cut off from the germ- 

 ring. But, we possess such excellent evidence, in the work of His, 

 Rauber, Whitman and others, that concrescence is, under normal 

 conditions, an important factor in the formation of the embryo, that 

 these experiments can not in their present form be taken to set aside 

 the great body of facts that favor that view. There is no occasion 

 to deney, totally, the operation of concrescence as a normal method 

 of body formation, because another means of body construction has 

 been, experimentally, found possible. The two possibilities may coexist 

 without being incompatible. 



Our views of embryonic growth have been extensively modified 

 within a few years by the experimental work of Roux and others 5 

 and many illustrations are afforded of how the egg may adopt itself 

 to greatly changed conditions and yet produce normally shaped em- 

 bryos. Under normal conditions, the embryo is built up, bilaterally, 

 by the simultaneous activity of both halves of the segmented egg, 

 but, the destruction of one half of the egg in certain stages, does not 

 prevent the formation of a bilaterally symmetrical embryo. This is 

 naturally smaller because the constructive material is reduced. 



Morphologists do not now hold that concrescence is the only 



