PROGRESS OF MICROSCOPICAL SCIENCE. 30 
above-mentioned dilatations was reached, a minute, beautifully clear, 
vesicular body floated out into the surrounding liquid; there was no 
sign of any adhesion to the uterine wall. He then goes on to say :— 
“The vesicles were as nearly as possible similar in size and 
appearance, and a description of any one of them will serve for all. 
Their shape was oval, the long diameter measuring about + inch, 
the short about 51; inch, and the outline being perfectly smooth and 
even. Under a low power of the microscope, the vesicle was dis- 
tinctly seen to be bounded by a primitive chorion or thinned-out 
zona pellucida. No trace of villi or projections of any sort could be 
detected on its surface. Besides this envelope, the wall of the vesicle 
was composed of what appeared a simple layer of flattened polygonal 
cells, closely lining the zona. But under a somewhat higher power a 
stratum of more deeply lying cells could, in some parts at least, be 
detected by focussing; moreover, a shadow at one place, midway 
between the poles of the oval, appeared to point to the possibility of 
the existence of a slight thickening at this part, although a well- 
defined shaded area was, in no sense of the word, visible. 
“T was led to imagine that the ‘ shadow’ in question, or rather the 
thickening to which it was probably due, would be caused by the first 
beginnings of a mesoblast at this situation. But nothing more could 
be made out in the fresh condition, and the little vesicles (at least two 
of them, for the others were of less value for the purpose of sections, 
owing to the blastoderm having shrunk away at various places from 
the zona, and presenting a crumpled, distorted aspect) were accord- 
ingly hardened in the usual way in very dilute chromic acid, stained 
with logwood and with carmine respectively, imbedded by the gum- 
process,* with the object of filling the cavity and thus supporting 
the enclosing parts and preserving them in their natural positions ; 
and finally sections were made across the long axis of the oval, and 
were mounted in glycerine and examined. 
“A glance at the sections is sufficient to show that the blasto- 
dermic vesicle is in the bilaminar condition. There exist within the 
zona pellucida, or primitive chorion, two distinct layers, the section of 
each forming a complete circle; the whole structure, therefore, in- 
cluded by the zona being formed of two separate vesicles, one within 
the other. The outer of these is of course the epiblast, the inner 
doubtless representing the hypoblast; we may speak, then, of an 
epiblastic and a hypoblastic vesicle. In none of the sections was 
there any trace of an intermediate layer of cells or mesoblast. The 
epiblast closely lines the zona throughout; but the hypoblastic vesicle 
is considerably smaller, so that except at one part, where it comes 
into closer proximity than elsewhere with the epiblast, it is separated 
from the latter by a considerable interval, filled in the fresh condition 
by a clear fluid. This fluid would seem to be of a different nature 
from that which occupied the cavity of the hypoblastic vesicle, for the 
coagulum produced in it by the action of the hardening liquid has a 
* A bad method for embryos; but I was at the time ignorant of Kleinenberg’s 
' excellent plan for effecting the same object. See Forster and Balfour, ‘ Elements 
of Embryology,’ p. 249. 
D2 
