648 The American Naturalist. [July, 
Mr. Hy. Orr {Qtiart. Jour. Micr. Sd., Dec, 1888) gives some de- 
tailed observations on the development of Anib ly stoma pinctatum (or 
A. bicolor) and of Rana halecina (or R. palustris), with special refer- 
ence to the central nervous system, and with notes on the hypophysis, 
mouth and appendages, and skeleton of the head. The central 
nerve-system first appears as a transverse epiblastic thickening, con- 
tinuous with paired elongated epiblastic dorsal thickenings. The first 
nerve-fibres of the brain appear on what was originally the internal 
surface of the primitive dorsal epiblastic thickenings. A subsequent 
development of nerve-fibres gives rise to a continuous ventral commis- 
sure and to the anterior and posterior commissures of the brain. 
Mr. Orr considers the balancers of Amblystoma as external gills of 
the mandibular arch, which have become metamorphosed into embry- 
onic organs of support. 
PHYSIOLOGY. 
Heart-sounds. — The well known experiment of Ludwig and 
Dogiel, who, by excluding the blood from the heart and presumably 
throwing out of function the atrioventricular valves, still heard the first 
heart-sound, is interpreted as evidence of the preeminently muscular 
character of that sound. Krehl,^ working in Ludwig's laboratory, finds 
yet stronger evidence of similar nature. Through the auricles he in- 
troduces a simple apparatus by which, at will, the atrioventricular 
valves may be held back against the cardiac walls, and thus thrown out 
of action or not interfered with. Observers, even physicians skilled in 
auscultation, are unable to perceive any differences, either in intensity 
or character, of the sound, whether the valves are in use or not. 
Bleeding the animal from the carotid does not interfere with the first 
sound until shortly before death, when the sound becomes feeble in 
accordance with the feeble beat of the heart. The experiments do not 
elucidate the question whether the heart-beat is a single twitch or a 
tetanus; if it be the former, the sound may easily be explained, as 
Ludwig himself has previously suggested, by the pulling or rubbing of 
the muscle fibres on each other. If the ventricular contractions be ex- 
cluded, a distinct but feeble auricular sound is heard. This may 
doubtless explain the "galop-rhythm," which is characterized by the 
