386 JAS. P. HILL. 



of the development in the two forms. Even now^ after nearly 

 two years' collecting, the material at my disi)Osal is by no 

 means large. It comprises altogether six stages, of which two 

 are post-partum. I take this opportunity of acknowledg- 

 ing my great indebtedness to the following gentlemen for 

 most generous aid in the by no means easy task of securing 

 the material on which this paper is based : — Messrs. J. B, 

 Cooper, A. G. Hamilton, A. M. Lea, Thomas Steel, and Dr. 

 A. E. Mills. To all these gentlemen I tender my sincere 

 thanks. 



To my friend Professor J. T. Wilson I am under a very 

 deep debt of gratitude for invaluable help in the way of sug- 

 gestion, criticism, and advice during the whole course of my 

 work. I am also indebted to my friend Professor C. J. 

 Martin for much kind advice. 



I have further to thank Mr. R. Grant, of the Physiological 

 Laboratory, for very great help in photography, and for assist- 

 ance in other ways. 



Lastly, I desire to thank my honoured chief, Professor W. 

 A. Haswell, F.R.S., for his uniform kindness and considera- 

 tion, and for much kindly interest in my work. 



Methods. — As fixing fluids picro-sulphuric and picro-nitric 

 acids were used. Most satisfactory staining results were got 

 by staining sections fixed on the slide by Mann's albumen 

 method, first with a weak watery solution of Renaut's hsema- 

 toxylic glycerine for eighteen to twenty-four hours, followed 

 by an alcoholic solution of eosin. Even better results were 

 obtained by the substitution of hsematein for hsematoxylin in 

 Renaut's formula. By this method of double staining the 

 foetal and maternal vessels can be beautifully differentiated. 



