410 C. M. CHILD 



constitutional development of a head or apical end occurs 

 (Child, '15 b, chap. IX). Moreover, the head is the chief organ 

 of physiological dominance or control, in short of physiological 

 integration (Child, '10b, 'lie, d, 12, '15b, chap. IX), it re- 

 presents the high end of the chief physiological gradient in the 

 body (Child, '13 b) and finally the graded series of head-forms, 

 ranging from normal to complete absence, may be determined 

 and controlled by many different agents and conditions, both 

 external and physiological. The present paper is primarily 

 concerned with the problem of head-development in its 

 relations, on the one hand, to these experimental and physio- 

 logical conditions and, on the other, to the hereditary constitu- 

 tion of the protoplasm. 



HEAD-FORM AND HEAD-FREQUENCY IN RELATION TO EXTERNAL 

 AND INTERNAL CONDITIONS 



It has been pointed out in various papers (Child, '11a, b, 

 '15 b, pp. 105 — 108) that a series of head-forms exists in the re- 

 constitution of pieces of Planaria dorotocephala. The terms of 

 this series have been separated for convenience into the groups: 

 normal, teratophthalmic, teratomorphic, anophthalmic, and 

 acephalic. These different forms of head, briefly described in 

 earlier papers and considered more fully below, evidently repre- 

 sent various degrees of inhibition of head-development, ranging 

 from the very slight degree of inhibition in the teratophthalmic 

 to the complete inhibition of the acephalic form. It has also 

 been shown that in material which is physiologically standardized 

 as far as possible as regards physiological age or size and nutri- 

 tive condition, pieces with anterior ends at a given level of the 

 body show a decrease in head-frequency with decrease in length 

 of the piece (Child, '11 a, b, d). Second, comparing pieces of 

 the same length from different levels of the body, we find that 

 head-frequency is highest at levels nearest the original head and 

 decreases as the level of the cut surface becomes more and more 

 posterior, back to the level of fission, where the head-frequency 

 suddenly increases again (Child, 'lib, d). Posterior to the 

 usual level of fission the different zooids are physiologically 



