291 
1888.] Prof. Wilhelm His on Animal Morphology. 
stand. A separating wall comes from the aortic bulb, another from 
the wall of the auricle, and the two unite with each other and with 
the primary ventricular diaphragm, so as to leave an opening from 
each ventricle to its auricle, as well as to its artery. 
As the heart has a bilateral origin, nothing could he more likely 
than the derivation of the two definitive halves from the primary 
ones. The two halves are connected longitudinally, and also the 
course of the blood has to follow two parallel streams. Instead of a 
longitudinal diaphragm between the two ventricles, observation shows 
a transverse one. This first diaphragm having been discovered and 
described by very good observers, the simplest way of separating the 
auricles and the aortic bulb seemed to be a direct continuation of 
this primitive septum. Many trials have been made to establish 
such a formation, hut they have all failed owing to farther obser- 
vations, and when at last a thorough examination of the heart in all 
phases of its development unveiled the whole of its complicated 
history, it was clear that the keenest intellect would have been 
impotent to find out a priori the peculiar ways of nature in framing 
this organ. 
The separation of the two ventricles in human embryos is finished 
during the fifth week, the embryos having at this time a length from 
10 to 13 mm. To anatomise in a satisfactory manner objects of 
these and even smaller dimensions, is only possible in some round- 
about way, and the first steps must he accomplished by the aid 
of the microtome. The object, after having been hardened and 
stained, is divided into a continuous series of fine sections. Micro- 
tomical manipulations are now so far perfected as to permit of the 
preparation of section of O'Ol mm. or less, and good instruments 
are everywhere to he found. Well-stained sections are so delicate 
in their appeaeance, and so rich in the finest details, that they exer- 
cise a strong attraction on every microscopical observer. We also 
owe to the study of such sections a very extensive acquaintance with 
important anatomical relations previously hidden from our eyes. 
But it is evident that, as sections are of only two dimensions, the 
most accurate observation, even of whole series, will not be sufficient 
to give a correct idea of a solid body. It is absolutely necessary to 
reintegrate the original body from its sections. 
The simplest way of combining the sectional views is by means of 
