The part of the penis enclosing the baculum, when not erect, is folded back as shown in Figures 1a and 1b. As a result the anatomically ventral surface faces upwards, or at least posterodorsally. The use of the term ventral in this account refers to the anatomically ventral side, that is to say to the side of the baculum facing the urethra.
The baculum in microtines consists of an elongate stalk, having a laterally, and to a lesser extent dorsoventrally, expanded base and an attenuate distal shaft. Usually, three digitate processes of cartilaginous material in which additional ossifications may occur arise from the terminus of the shaft. The proportions and curvature of the stalk vary as do the proportions of the terminal ossifications to each other and to the stalk. In some species one or more of the digital processes are frequently completely unossified.
Figure 1. The baculum in Microtus ochrogaster—orientation and variation with age. a. Diagram of a sagittal section of the posterior half of a vole, natural size. The penis, containing the baculum (in black), extends ventrally from a point posterior to the pubic symphysis (stippled), along the body wall, and bends posteriorly at the distal end. b. Distal end of penis (× 2) showing baculum (in black), the urethra (solid lines) adjacent to the baculum, and the corpus cavernosum (broken lines) proximal to the baculum. c. Oblique view of the cross section of penis (× 4) shown in Figure 1 b. The thick-walled urethra lies ventral to the curved corpus cavernosum. A thinner-walled blood-vessel lies dorsal to the corpus cavernosum. The anatomically ventral side of the baculum, in the normal non-erect penis shown, is seen to face dorsally. d. Graph showing the relationship between size of baculum, size of animal, and development of digital ossifications. Circles show presence of ossification in stalk only; circles enclosing dots indicate presence of secondary ossification in median process also; large dots indicate the addition of tertiary ossification in one or both of the lateral digitate processes.
Preserved specimens of Microtus arvalis, Microtus agrestis, Microtus orcadensis, Microtus nivalis, Microtus guentheri, Microtus subterraneus, Clethrionomys glareolus, and Ellobius lutescens were provided by Prof. Robert Matthey of Lausanne, Switzerland. J. Knox Jones, Jr. carefully saved the bacula with specimens of Microtus fortis and Clethrionomys rufocanus from Korea. Dr. W. B. Quay, Department of Zoology, University of California, supplied specimens of Synaptomys cooperi, Phenacomys intermedius, and Microtus oregoni. Dr. Franklin Sturges and Mr. John W. Goertz, Museum of Natural History, Oregon State College, Corvallis, have provided specimens including bacula of Clethrionomys occidentalis, Microtus oregoni, and Microtus townsendii. Dr. Randolph L. Peterson and Mr. Bristol Foster, Royal Ontario Museum of Zoology, Toronto, Canada, provided specimens of Phenacomys intermedius. Dr. J. N. Layne, University of Florida, Gainsville, Florida, presented me with a baculum of Microtus parvulus.
I am indebted to all of these persons for their aid, and to various collectors for the Museum of Natural History, who preserved bacula with specimens. Many of these specimens were obtained through the assistance of the University of Kansas Endowment Association and the National Science Foundation.
METHODS
Bacula were obtained from fresh specimens, specimens preserved in alcohol or formalin, and dried study skins. The processing of bacula has been discussed by Hamilton (1946), Friley (1947), White (1951), and Dearden (1958). The methods used to preserve bacula for my study differed some from any of those reported. The terminal part of each penis including the baculum imbedded in the glans penis was removed in its entirety and placed in a vial. The catalogue number was kept with each specimen at all times. A two per cent solution of potassium hydroxide was added. All specimens were examined at least once a day. If tissues other than the glans penis were present they were removed with forceps when softened usually at the end of one day. Several drops of Alizarin red-S stain in a saturated alcoholic solution were added to the 3 to 5 ccs. of KOH solution in each vial. Solutions were replaced if they became turbid enough to obstruct observation of the clearing penis. After one day the solution containing stain was removed and replaced with two per cent KOH solution without stain. When the glans became sufficiently cleared that the stained baculum could be seen easily, the solution was replaced by glycerin in which clearing was completed. The time required for the entire process varied from one day to more than two weeks depending on the size of the specimen and on its condition. Fresh specimens clear more rapidly than dried specimens, and those that are dried more rapidly than those that are preserved. A three or four per cent solution of hydroxide will hasten the process, but more frequent observation is required to prevent excessive maceration.