DR. NELSON ON THE REPRODUCTION OF THE ASCARIS MYSTAX. 
583 
The division of the nucleus (fig. 72 k) is immediately followed by that of its cell, 
the embryonic vesicle (fig. 73 a) ; and thus two embryonic vesicles are formed, each 
containing a separate nucleus or spot (fig. 73 A-). 
As soon as this has occurred, the two cells (fig. 73 a) are seen to separate and 
approach the opposite sides of the yolk ; a portion of the yolk membrane (fig. 73 o) 
is protruded outwards, by the application of one of the embryonic cells against it. 
At first this protrusion is very slight (fig. 73 o), but by the continued movements of 
the vesicle (fig. 74 o) it becomes more and more increased, till at last the yolk 
assumes an oblong shape, with a constriction about the middle (fig. 74 o). The con- 
striction gradually deepens, till at last two yolks are formed (fig. 75 o) by the sudden 
division of the investing membrane. 
1 have repeatedly watched this process as it occurred under the microscope, and 
found that while the division of the embryonic vesicles takes from five to ten hours, 
the division of the yolk does not take more than thirty minutes. 
The separation of the yolk into two parts is, I think, entirely a mechanical effect, 
and not produced by vitality inherent either in the yolk-granules (fig. 75 n) or 
membrane (fig. 75 o). For, besides the rapidity of its accomplishment, I have ob- 
served that the embryonic vesicles (figs. 73, 74 a) continue during the progress of 
the division to revolve round and round in circles ; the one moulding the newly pro- 
jected portion of the yolk membrane (figs. 73, 74 o) into a spherical form, while the 
other prevents the original part from collapsing. 
Sometimes, when the formation of a yolk has been prevented by immersion in 
some preservative fluid, the division of the embryonic vesicle still takes place 
(fig. 71 a), and the two are generally seen occupying opposite ends of the egg, but 
without any membranous or granular investment. 
As soon as the yolk has divided into two (fig. 75 o), a pause occurs. The two 
embryonic vesicles (fig. 75 a) remain stationary, their nuclei (fig. 75 k) subdivide, 
they themselves elongate, and ultimately separate into two each. Thus four em- 
bryonic vesicles are formed, two within each yolk mass, which, by the repetition of 
the same process, is redivided into four (fig. 76 o). 
Occasionally, when one embryonic vesicle divides more rapidly than its fellow, 
three yolk masses are produced ; but this is rare, and not usually the case. 
As this process is repeated from time to time, the number of the yolk masses 
increases from 4 (fig. 76 o) to 8, 16, 32, 64, 128, 256, &c., till they (fig. 77 o) become 
so numerous and so minute as to appear like granules (fig. 78 o) ; yet each granule 
(fig. 78 o) is composed of a nucleus, an embryonic cell, yolk substance, and yolk- 
membrane. 
From the immense amount of subdivision and the number of interspaces caused 
by the spherical form of the granules, the whole egg is filled with them, giving it a 
dark or opake appearance (fig. 78). 
A membrane appears to form on the external surface of this opake mass, corre- 
