DR. NELSON ON THE REPRODUCTION OF THE ASCARIS MYSTAX. 
581 
inch in diameter; its nucleus, or as it is commonly called, the germinal spot (figs. 
64, 65 tf), is of an inch, and the contained nucleolus of an inch 
(figs. 64, 65 A-). 
After the rupture, the nucleus and nucleolus (fig. 67 a, h) are of exactly the same 
size, 4 - 0^0 o th and g - o^Q - Q -th of an inch respectively. 
But soon the nucleus, which is at first solid (fig. 67 «), begins to enlarge, swells 
up, and constitutes a transparent cell (fig. 68 a ) ; while the nucleolus remains of the 
same size (fig 68 h), forming, in short, an embryonic vesicle and spot. 
As soon as the embryonic vesicle (fig. 68 a) begins to form, a membrane separates 
from the internal surface of the chorion (fig. 69 f), and gradually contracts (fig. 69 o) 
on the embryonic granules (fig. 69 w), till a perfect sphere is formed, whose breadth 
is nearly equal to the lesser internal diameter of the ovum (fig. 70 o). 
How, or from what this membrane (fig. 70 o) is formed, I cannot speak with cer- 
tainty, further than that it separates from the inner surface of the chorion (fig. 69 f), 
and contracts on the embryonic granules (fig. 69 n) to form a true or embryonic 
yolk, exactly in the same way that it does (Plate XXVIll. fig. 56 o) in the false ovum 
(fig. 56) to form the opake or false yolk (fig. 56 c). 
It is probable, therefore, that the formation of the yolk-membrane (Plate XXIX. 
fig. 70 0 ) is a physical process, unconnected with the fertility, or the individual 
vitality of the egg, as it takes place exactly at the same time, and in the same manner 
in the unfecundated or sterile ovum, into which the spermatic particles have never- 
entered. 
When the membrane of the embryonic yolk first separates from the inner surface 
of the chorion, it encloses not only the embryonic granules (fig. 69 w), vesicle 
(fig. 68 a) and spot (fig. 68 A'), but likewise the clear fluid in which they float; al- 
though, as contraction goes on, this fluid passes through the membrane (fig. 70 0 ) 
and occupies the space between it and the external envelopes (fig. 70 f). It there- 
fore acts the part of a sieve, allowing the fluid to pass, but retaining the granules, 
and bi-inging them within the influence of the embryonic vesicle (fig. 70 a). 
The embryonic yolk is at first large and irregular in shape (fig. 69 o), but it soon 
becomes perfectly spherical (fig. 70 0), -g^th of an inch in diameter, enclosing an 
embryonic vesicle and spot, whose sizes are -2^otb and gwoth of an inch respectively 
(figs. 70 a, Ti). 
At this period the egg (fig. 70) is oval, its longer diameter being -g^th of an 
inch, and its shorter g^i^th of an inch ; its membranes are firm and resisting 
(fig. 70 f ) ; and with this amount of organization it is expelled from the body of the 
mother. 
The perfect ovum, therefore, consists of two or three homogeneous membranes, 
united to form one oval shell (fig. 70 f), some limpid fluid (fig. 70 m), a spherical 
embryonic yolk membrane (fig. 70 o), embryonic granules (fig. 70 n), an embryonic 
vesicle (fig. 70 a), and its nucleus, the embryonic spot (fig. 70 k). 
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