A Bilateral Division of the Parietal Bone in a Chimpanzee; with a Special Reference to the Oblique Sutures in the Parietal — Key Ideas to Explore

Comparative Anatomy and the Parietal Suture

Hrdlička builds his argument by juxtaposing his chimpanzee specimen with Ranke’s findings in orangs and with human cranial anomalies. He notes that Ranke found 13 incomplete parietal divisions among 245 orang skulls, all involving a remnant of a horizontal “parietal suture” ending in the coronal suture. The author also cites Putnam’s cases of oblique sutures co-existing with horizontal ones on opposite sides. This comparative method allows Hrdlička to propose that such bilateral divisions, though rare, point to a common developmental origin. He is careful to distinguish true parietal sutures from those simulating Wormian or fontanel bones, emphasizing that the extent and form of the division are key diagnostic features.

Theories of Ossification and Supernumerary Centers

Hrdlička entertains three possible explanations for oblique parietal sutures: persistence of an early embryonic suture, coincidence of anomalous conditions, or a supernumerary third center of ossification. He is cautious about the third theory, stating he is “by no means ready to defend” it, yet he presents a Peruvian skull with bilateral quadrangular separate pieces that are too large and singularly outlined to be simple Wormian bones. The measurements—20 mm by 12–21 mm on the left, 25 mm by 11–15 mm on the right—illustrate the ambiguity. Hrdlička calls for further observation, particularly on infant and embryonic bones, to resolve these diagnostic difficulties.

Rejecting Fracture and Wormian Explanations

Returning to the chimpanzee skull, Hrdlička firmly dismisses the possibility that the divisions are fractures or boundaries of Wormian bones. He asserts that “there is nothing about the sutures, or the divided pieces, or the neighboring bones” to suggest such explanations, and that no analogies exist in records for man, apes, or lower animals. The left-side division, he argues, would alone be acceptable as an instance of the “parietal suture,” with its anterior extremity and course matching known patterns. This decisive rejection clears the way for his conclusion that the specimen presents two genuine examples of parietal division.

Structure of the Argument and Use of Illustrations

The article’s structure moves from specific case to general theory and back again. Hrdlička begins with the chimpanzee, expands to comparative data from orangs and humans, discusses theoretical mechanisms, and then returns to the original specimen for final diagnosis. This circular structure mirrors the investigative process. The text repeatedly references figures—such as Fig. 1 showing Ranke’s orang and Fig. 6 depicting the Peruvian skull—though the excerpts do not include the illustrations themselves. These references anchor the verbal descriptions in visual evidence, a common practice in anatomical bulletins of the period.

Readers should approach Hrdlička’s bulletin as a focused piece of comparative anatomy, not a general survey. The author’s willingness to entertain multiple hypotheses—while ultimately committing to a single interpretation—offers insight into early twentieth-century scientific reasoning. The text rewards attention to its precise measurements and cautious phrasing, particularly where Hrdlička distinguishes between what can be asserted and what remains uncertain.

That old chimpanzee skull study stayed with me—not for its bones, but for how gently it wondered about the tiny seams that make us whole. It reminded me of watching insects in Fabre's Book of Insects — Text and Context, where patience turns into quiet awe. Some books leave a soft echo, like a room you’ve just stepped out of, still warm.