440 Development and Vascularization of the Testis 
epididymis. He also describes a capillary network around the tubules. 
Astley Cooper has investigated a capillary plexus covering the internal 
surface of the tunica albuginea which he has termed the tunica vasculosa. 
But beyond this there is little to be found in the literature of the subject. 
The vascular units which Professor Mall and his students have shown to 
be present in several organs and which they assume are to be found in 
all organs, have not as yet been demonstrated in the testis, and, indeed, 
aside from what has been cited above, nothing is known of the arterial 
or venous supply of the male sex gland. Dr. Mall has frequently dem- 
onstrated the presence of certain units of the blood system which may 
or may not be peculiar to the organ in which they are found and which 
correspond to the histological or structural wnit of the organ. These 
units are composed of small branching blood vessels which pass into 
capillaries and the blood from which is collected into small veins. This 
theory of vascular units may be briefly summed up in the statement 
“similar blood supply to similar histological units.” These vascular 
units have been proved to be present in the liver,’ spleen,’ and adrenal,” 
but in the testis of the pig I can make out no definite units. In man, 
however, the lobular arrangement is less complex, and results have been 
so encouraging that probably these units will be shortly discovered. 
Methods and Material—The necessity of clearly understanding the 
development of the circulatory system in the earliest embryonic stages 
in order to properly interpret the course of the blood stream in the adult 
organs has been frequently emphasized. Accordingly, in attempting 
this research frequent use has been made of embryonic material. Em- 
bryo pigs, many of which were alive when delivered at the laboratory, 
were injected and cleared, and, after tracing the development of the 
circulation in these stages, the investigations were completed with adult 
material. For adult human testes I am indebted to Professor MacCal- 
lum of the pathological laboratory. To Professor Brédel I also wish 
to express my appreciation for several valuable specimens of human 
embryonic testes as well as for his helpful suggestions in making the 
illustrations. For the courtesies of their laboratory I wish to. thank 
Professors Wiedersheim and Keibel of Freiburg. 
All injections of embryonic material were made with India ink. In 
1 Mall, F. P.: A Study of the Structural Unit of the Liver. Am. Jour. Anat., 
WoOlsaV suNOe. 
?Mall, F. P.: The Structure of the Spleen. Johns Hop. Hosp. Rep. 
’ Flint, J. M.: The Blood Vessels, Angiogenesis, Organogenesis, Reticulum 
and Histology of the Adrenal. Johns Hop. Hosp. Rept., Vol. IX. 
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