538 ON INCREASE IN SIZE [pt. iii 



embryonic growth, and the few data that do exist on the subject 

 will be presented in the section on comparative susceptibility of the 

 embryo at the different stages in its development. The section on 

 biophysical phenomena also contains information on cognate points. 

 The process of cell-division, as such, is of course influenced by a 

 great variety of factors and substances, as the following table shows : 



Table 77. 



Substances which accelerate 



cell-division Authority 



Heat ... ... ... ... ... Laughlin 



X-rays ... ... ... ... ... Gilman & Baetjer 



Radium emanation ... ... ... Packard; Bohn; Shumway; and 



Buddington & Harvey 



Th>Toiodin ... ... ... ... Richards 



Adrenalin ... ... ... ... Chambers 



Alcohol Calkins; Woodruff 



Potassium hydrogen phosphate ... Woodruff 



Potassium sulphate ... ... ... „ 



Potassium bromide ... ... ... ,, 



Oxygen ... ... ... ... Godlevski 



Sodium and potassium hydroxide . . . Richards ; Loeb 



Pilocarpine ... ... ... ... Richards 



Ammonium hydroxide ... ... ,, 



In most cases, however, the effects produced by these agents are 

 complicated, and the original papers should be referred to. Naturally, 

 all these substances and factors exercise a depressant action on cell- 

 division if they are applied in too brutal a manner. Moreover, though 

 they tell us a certain amount about the nature of mitosis, they do 

 not much assist in the understanding of the metazoal growth-process, 

 organising mitoses as it does on the large scale. As Richards says, " The 

 advantage gained in the segmentation-stages may later manifest itself 

 in more vigorous larvae than in more rapidly developing ones ", so that 

 agents which accelerate cleavage may not accelerate hatching-time. 

 The study of these agents has not led so far to any great advance in 

 our knowledge of the essential nature of growth and development. 



2-19. The Effect of Hormones on Embryonic Growth 



As regards the effects of endocrine organs upon embryonic growth, 

 practically nothing is known. Willier, who made chorio-allantoic 

 grafts of thyroid in chick eggs, observed that the host embryo was 

 always smaller, and in some cases as much as one-third smaller, than 

 the control. Hanan later, after a tliorough study of the difficult 

 question of appropriate dosage, injected 1/600 of a mgm. of thyroxin 



