402 JOHN LEWIS BREMER 



can be traced in some places. (Hill, fig. 12.) In both the fourth 

 and the seventh months a vascular unit can be made out, as the 

 veins are arranged in a network around the terminal arteries in 

 the usual manner. Hill mentions "vascular units which corre- 

 spond to units of structure and which repeat themselves similarly 

 throughout the organ," but does not state what these units of 

 structure are; I have found in the testis of seven months and in the 

 adult that the structural unit corresponding to the vascular unit 

 consists of a number of coils of a single tubule enclosed in a com- 

 partment, as described earlier in this paper. The border veins 

 of the unit lie in the connective tissue which surrounds the con- 

 volutions, the terminal artery pierces the compartment. It is 

 probable that the terminal artery is the causative factor in this 

 unit ; the portion of a tubule situated nearest to the artery would 

 be more favorably placed for growth and consequent convolu- 

 tion, than the portions of the tubule further from the blood supply; 

 the convolutions would therefore form around the arteries, one 

 set for each terminal artery. These units are quite large, readily 

 visible to the naked eye; the capillaries surrounding the tubules 

 are therefore of considerable length. As structural units they are 

 not typical, like the unit of the lung or of the salivary glands, since 

 they only very occasionally represent the terminal, blind ends of 

 the secreting or active portion of the gland, and since there is no 

 constant relation between the terminal artery and the channel 

 through which the secretion leaves the unit. One tubule passes 

 through several units, but all the convolutions within a unit be- 

 long to the same tubule. 



The peripheral layers of the original capillary networks, both 

 arterial and venous, interdigitate with the peripheral portions of 

 the cords, while these portions are still attached to the peritoneal 

 epithelium from which they grew. When this connection is lost 

 and the peripheral ends of the cords have been absorbed into the 

 network, the peripheral vascular networks remain, and become in- 

 corporated in the vascular layer of the tunica albuginea ; the main 

 vessels are given their prominence by their direct connections 

 with the main radial vessels within the testis. In the ovary, this 



