the Embryo of Purpura lapillus. 
19 
increase and development thus seem to take place at their 
expense ? 
This is the problem to which MM. Koren and Danielssen 
applied themselves, and of which their solution presents phy- 
siological difficulties scarcely inferior to those of the problem 
itself. Their account of the process is briefly as follows : — 
Each of the numerous bodies originally aggregated in the 
ovigerous capsules, is regarded by them as a true ovum ; of 
which the vitellus undergoes a cleavage, which is at first 
regular, but subsequently irregular, dividing it into from six 
or seven, to sixteen or eighteen cleavage-segments. These 
segments then agglomerate into a compact mass, in which, 
however, the clusters formed by the separate ova may be dis- 
tinguished for some time. At a later period, they describe this 
conglomerate as presenting manifest subdivisions, which 
become more sharply circumscribed, and project from the 
remainder of the mass. These projecting groups soon take 
a cylindrical or pyriform shape, and are fixed to the rest by 
means of a peduncle. The microscope shows them to be 
composed of a delicate ciliated membrane, enclosing a mass 
of ova ; a transparent substance exudes from the two sides of 
the peduncle, upon which fine cilia appear (the foot) ; and at 
the base of this same peduncle, the first traces of the lobes 
are distinguishable. Finally, many of these pyriform bodies 
become detached from the mass, and rotate upon themselves ; 
these are the embryos." Thus, according to these observers, 
a number of ova, varying from three or four to sixty or more, 
coalesce to form each perfect embryo. 
But they have also ob/^erved another phenomenon, which 
they consider as altogether abnormal ; namely, the develop- 
ment of an embryo from a single ovum, which does not become 
fused into the conglomerate mass. Such an embryo, they say, 
is always to be found in each capsule, up to the eighth week 
of its development ; and it is at once known from the rest, not 
only by its small size, but by the very imperfect development 
of its organs, the ciliated lobes and foot being the parts first 
and most completely evolved, and the other organs being 
nearly or entirely abortive ; this inequality giving a strange 
and (at first sight) almost incomprehensible appearance to the 
* monster.' Hence, then, they conclude that for the viability 
of the individual organized, more than one ovum is necessary ; 
and despite the regularity and the vivacity observable in the 
young product of the single ovum, we see that its development 
remains in the highest degree incomplete. This single ovum 
had in fact undergone all the stages of cleavage, and to all ap- 
pearance united all the anatomical and physiological conditions 
c 2 
