582 
DR. NELSON ON THE REPRODUCTION OF THE ASCARIS MYSTAX. 
Compare the fecundated (fig. 70) with the unfecundated ovum (Plate XXVIII. 
fig. 56), and one is immediately struck with the immense difference that exists be- 
tween them. 
In the false ovum there is no embryonic vesicle, no embryonic spot ; while the 
substance that does exist, is apparently the colouring matter of the vitelline granules, 
collected into a structureless yolk (fig. 56 c), surrounded by a membrane (fig. 56 o), 
and the whole enclosed in a granular chorion (fig. bQf) instead of a laminated shell 
(Plate XXIX. fig. 70/). 
The formation of the embryonic yolk membrane is not the effect of fecundation 
(fig. 70 o), because we see one produced in the sterile ovum ; but after the entrance, 
swelling up, and solution of the spermatic particles, certain other changes are pro- 
duced within the ovule, which do not occur otherwise. 
The spermatic particles, by penetrating into the ovule, exert over it an influence 
of three distinct and somewhat opposed kinds. 
First. A preservative effect, preventing the decay, disappearance, and blending 
together of the vitelline granules, the germinal vesicle and spot. 
Secondly. A destructive or solvent influence, by which the vitelline granules and 
germinal vesicle are, after a time, gradually dissolved. 
Thirdly. A power of transformation, by which the vitelline are changed into 
embryonic granules. 
The preservative, destructive and transformative influences commence, as we have 
seen, with the union of the spermatic particles and ovule ; they are conferred by the 
spermatic particles on the ovule, which continues to exist, while the sperm is de- 
stroyed by the act ; and lastly, they appear all three to be of a purely chemical nature. 
These properties once acquired, continue not only throughout the whole life of 
the creature, but remain after the death of the individual. 
To one or other of these influences may all the changes that take place in the 
living body be ascribed, with the exception of those that are referrible to life alone. 
But before entering on the consideration where life commences, and in what part 
it resides, it is essentially necessary that we make ourselves acquainted with the 
changes it effects in the ovum, by which the egg is transformed into an embryo, in 
all respects similar to the parent Ascaris ; like it, capable of voluntary motion, assi- 
milation, and the power to produce other ova. 
These are most beautifully seen in the egg of the Ascaris mystax but as they 
have been already described by far abler authors, 1 shall confine myself to a very 
brief outline of the changes as they occurred under my own observation. 
The first alteration that the ovum undergoes is the division of the embryonic spot 
(Plate XXIX. fig. 72 k), and elongation of the embryonic vesicle (fig. 72 a). 
This division is sometimes seen even before the germinal vesicle has disappeared 
(fig. 65 k), but does not take place normally till after the formation of the true or 
embryonic yolk (fig. 70 o). 
