Dr. A. Korotneff on the Embryology of Hydra. 435 



would be a continuous layer of nerve-cells. This theory has 

 already been discussed by several authors, and generally re- 

 garded as erroneous. My observations at the first glance seem 

 to give some support to it. We actually find in Hydra 

 aurantiaca the complete disappearance of the primary epiblastj 

 in Hydra fusca, on the other hand, the cells which form the 

 egg-capsules also take part in the formation of the definitive 

 ectoderm. To be logical, therefore, we should have to 

 assume that in one species the ectoderm is formed of nerve- 

 cells, and that in another it consists of epithelium-cells. In 

 my own opinion we may regard the metamorphosis of Hydra 

 (its histolysis) as a direct result of external influences which 

 are quite sui generis and furnish us with no grounds for 

 any homologies. In the lower animals, the Coelenterata espe- 

 cially^ the principle of the formation of the organism out of 

 definite embryological lamelke cannot be carried through to its 

 ultimate consequences ; the function of the lamella? is here not 

 so specifical as in higher forms. An example of this is 

 furnished by the formation of the nervous system. The ecto- 

 derm is certainly to be regarded as the starting-point of its 

 formation ; but in the Mollusca its origin from the mesoderm 

 is almost everywhere demonstrated. Further, the investiga- 

 tions of the brothers Hertwig have shown that in the Coelen- 

 terata (Actiniae) the entoderm possesses quite independent 

 nerve-elements, which may probably have an entodermal 

 origin. Without venturing more deeply into this subject, I 

 would affirm that in the development of the lower organisms 

 we find exceptions to and modifications of the course of action 

 which is to be regarded as the rule ; and these probably are to 

 be considered the results of adaptation. It is certainly indis- 

 putable that the adaptability is much more considerable in the 

 lower organisms than in the higher ; and for this reason exter- 

 nal influences may completely transform the organism itself 

 and its developmental history. It is only from this stand-point 



Hyd 



Hydra can be understood. I regard 



have no very definite part to perform in the plastic develop- 

 ment of the organism. And if the outer layer of the morula 

 becomes altered, and, instead of taking part in the develop- 

 ment of the organism, forms a shell or skin for the embryo, 

 this also is to be interpreted as a process of adaptation. 



