322 C. M. CHILD. 



condition are all factors in the capacity for head formation, 

 pieces which are to be directly compared must be of the same 

 size, except of course where the size factors are under consideration, 

 they must be taken from the same region of the body except 

 where the regional factor is to be analyzed and from individuals 

 in similar physiological condition except where different physio- 

 logical conditions are being compared. As noted above a re- 

 lative measure of physiological condition is possible; it is also 

 possible to control the size of the piece and the region of the body 

 from which it comes within certain limits, especially in the 

 pharyngeal region and near the head where localized morpholog- 

 ical landmarks exist. 



The method of experiment which I have found most satis- 

 factory is to compare considerable numbers of similar pieces 

 rather than individuals. For a temperature experiment for 

 example, animals of the same size and as nearly as possible in the 

 same physiological condition are taken as the basis: from these 

 pieces as nearly as possible of the same size and from the same 

 region of the body are cut and a number, usually fifty, of these 

 pieces is placed in one temperature and a like number in another. 

 For an alcohol experiment fifty pieces obtained in a similar 

 manner are placed in alcohol and fifty in water at the same 

 temperature and under otherwise similar conditions. 



By taking into consideration the factors of size of piece and 

 region of the body it is possible to cut series of pieces which will 

 give 100 per cent, or very nearly of normal heads, or on the other 

 hand 100 per cent, or very nearly of headless forms, or we can 

 obtain series which in consequence of unavoidable differences 

 in size and region and differences in the physiological condition 

 of the animals from which they were taken, produce a certain 

 percentage of several of the types described above. In all 

 these cases we can compare the results under the control con- 

 ditions with those occurring under other definitely known con- 

 ditions. 



For example, pieces above a certain size from the anterior 

 region of the body, which give 100 per cent, or nearly of normal 

 heads in well aerated water at a temperature of 2OC. show a 

 large percentage of teratophthalmic forms in water at ioC. or in 



