AND THE DEVELOPMENT OF THE MYRIAPODA. 
105 
condition. They have always exhibited the same yellow colour, with only a slight 
difference of opacity. At a later stage of development the whole appear to be in- 
closed in a distinct membrane, the membrana vitelli (fig. W.d), and even at this 
early period (fig. 8.) the membrane may be regarded as in the course of formation, 
as seems to be indicated by the fact, that the cells always cohere together when the 
specimen has been placed in spirits of wine, and afterwards allowed to remain for 
some time in water. In these cases the ovisac becomes distended by the imbibition 
of fluid, as is proved by the existence of a clear transparent space between the interior 
of the ovisac (figs. 7 to 11. d) and the yelk (e), the cells of which do not separate, but 
together retain the form of a single mass unaltered, even when pressed in different 
directions between plates of talc beneath the microscope. When the ova are a little 
further advanced (fig. 9.), the same yellow-coloured graniform cells compose the 
yelks ; but they are a little darker in colour, larger, and more distinctly exhibit a 
granular structure in their interior : they closely resemble in form and structure the 
vesicles of the yelk of the higher animals, as described in the excellent researches 
of Dr. Martin Barry*. The membrane of the yelk (d) at this period is more strongly 
marked, and exhibits a distinct outline, when treated as above described ; and the 
germinal vesicle is much more apparent (a, b). At this stage of the ovum, the outline 
of the vesicle is more distinct than in the previous stages, and this body is slightly 
larger. The macula (a) is of a perfectly globular form, and is apparently covered 
by a separate membrane^. It is very distinct, and is formed by an aggregation of 
minute cells, or vesicles, surrounded by fluid. In a more advanced stage of deve- 
lopment the vesicle is surrounded at a little distance by an outer ring, which exists in 
all the ova in the succeeding stages, but is not seen in the first stages of the ovum. 
When a perfect egg that has passed into the oviduct, and is ready to be deposited, 
has been placed in spirits of wine, and is afterwards examined beneath the microscope, 
there are seen on the outside of its opake shell one or more large spots (Plate IV. 
fig. 5. b. c c), which appear to be occasioned by some circumstance connected with 
the formation of the shell in the oviduct or ovary. These spots are large and oval, 
are formed by concentric rings, and appear to be the result of incomplete depositions 
of the different layers of material of which the shell is composed. In the vicinity 
of the spots within the egg I have usually found some aggregated oil-globules. The 
yelk is of a light yellow colour, and occupies nearly the whole of the interior, there 
being only a very small space around it, and a very slight quantity of albumen. The 
form of the yelk within the shell, when the egg has remained for some time in spirits 
of wine, is irregular. At one end it becomes obtuse, and rounded ; at the other 
incurved, and a little pointed. On one surface it is large and convex, and on the 
opposite concave or excavated. In the middle of the concavity is the transparent 
globular vesicle, the proper germ vesicle, considerably enlarged, and presenting the 
* Philosophical Transactions, 1838, 1839, 1840. 
t See Wagner’s Physiology, translated by Dr. Willis, Part I. 1841, p. 41, note 48. 
