176 C. M. CHILD. 



pie instead of ectodermal, neurogenic, myogenic, mesenchymal, 

 entodermal, substances, etc., we have head-forming and tail-form- 

 ing substances, pharyngeal substances, hydranth-forming material, 

 stolon-forming material, etc. How do these substances arise 

 from the substances supposed to exist in ontogeny. A head or 

 tail of a planarian or the hydranth of a hydroid is a very com- 

 plex structure which has developed from regions more or less 

 widely separated in the egg. Is the head-forming substance 

 made up of all the different substances which were concerned in 

 the original development of the head or is it something new ? If 

 it is the sum total of the ontogenetic substances then it must in 

 itself be organized, i. t\, the different parts of which it consists 

 must possess characteristic positions with reference to each other 

 and we have another organization to account for. Moreover, 

 why should such an enormous amount of head- and tail-forming 

 substances as must exist for example in planarians be left over 

 after development is completed ? What is their function in the 

 normal animal or are they without function ? If the latter is the 

 case it looks very much as if provision were made in the normal 

 animal for regulation, in other words as if regulation might be 

 after all an adaptation. 



Polarity is regarded as identical with a postulated gradation of 

 the various materials. The head-forming materials in planarians, 

 for example, are supposed to decrease from the anterior end 

 backward, the tail-forming materials in the opposite direction, 

 the pharynx-forming material from the middle toward both ends, 

 etc. This idea followed to its logical conclusion certainly does 

 not assist us in comprehending organic structure, but rather in- 

 creases the difficulty. How did this gradation come about and 

 why should it exist ? As we have seen, the various head-forming 

 substances were widely separated in the egg. How and why 

 have they combined in this peculiar manner? Why does not a 

 small piece from the region of the body just behind the head in 

 Planaria always give rise to heads at both ends, since the head- 

 forming substance is greatly in excess at both ends as compared 

 with the tail -forming substance ? Why does not the pharynx in 

 such a piece arise at the extreme posterior end since the pharynx- 

 forming substance is more abundant there than elsewhere in the 

 piece ? 



