584 
DR. NELSON ON THE REPRODUCTION OF THE ASCARIS MYSTAX. 
spending to the internal surface of the chorion ; the production of which is attended 
with a loss of some of the most superficial granules. 
Next, a depression (fig. 79 p) of this membrane takes place on one of the sides. 
The depression is at first slight (fig. 79 j»), but it gradually increases, forcing some 
of the granules before it, while others disappear by solution into a limpid fluid, which, 
passing through the membrane, occupies the space between it and the shell (fig. 79 r). 
A hemispherical mass is thus produced, but the central portion continues to 
advance, forming first a cup-like depression (fig. 80 7 ?), and, ultimately touching the 
membrane of the opposite or convex side, unites with it, to produce a thick circular 
ring (fig., 81 p). 
This fleshy ring (fig. 81 /?) soon presents a constriction at one point (fig. 82 a). 
The constriction, by deepening, divides the ring, which is thus transformed into a 
cylindrical worm, bent round, so that the two ends are in apposition (fig. 83 s ) ; and 
covered externally with the membrane (fig. 83 p),now become thick, while internally 
we still see nothing but granules (fig. 83 o). 
As the body elongates, the two ends overlap, and are seen to be pointed. At first 
the overlapping is slight (fig. 83 s), but it gradually increases (Plate XXX. figs. 84, 85, 
86 s), till at length the little worm forms nearly two turns of a spiral (figs. 87 , 88 p), sur- 
rounded on all sides by fluid (figs. 87 , 88 r) and the chorion, or shell (figs. 87 , 88 f). 
By rupturing the egg, the embryo worm is set free (fig. 89), and is seen to possess 
the three-lobed mouth (fig. 89 s), peculiar to the genus, and a very thick cuticle 
(fig. 89 p) enclosing a number of untransformed granules (fig. 89 0 ). 
The development of the embryo is best observed by placing the females entire in 
spirits of turpentine for a fortnight or three weeks ; at the end of which I have 
found the ovaries distended with ova, all of which contained young worms, not only 
fully developed (figs. 84 to 88 ), but alive, endeavouring their utmost to rupture the 
chorion, by rolling themselves up into a tight spiral (fig. 88 p), and then suddenly 
reversing the coil. 
Let us now return to the consideration as to which of these changes are vital, and 
which physical. 
The most remarkable, as well as most apparent change that takes place in the 
ovum subsequent to penetration, is the division of the yolk, a phenomenon, which, 
although peculiar, seems to be entirely mechanical. 
For the yolk membrane when at rest, as seen before the division of the yolk, 
assumes the spherical form, by its own molecular attraction (Plate XXIX. fig. 70 o ) ; 
but when drawn out by the embryonic vesicles, acquires first a cylindrical shape, 
then that of an hour-glass, because that part of the membrane occupying the centre 
of the cylinder, having nothing to keep it distended, collapses ; while the two ends 
are prevented from doing the same by the constant movements of the embryonic 
vesicles (fig. 74 a). 
When the hour-glass form (fig. 74 o) has been once attained, the molecular attrac- 
