EMBRYOLOGY OF THE TURTLE. Part III. 
latter, and, still later, plunges into its substance, till the whole yolk mass becomes 
a great network of bloodvessels, (Pl. 18, fig. 4,) a vascular area, hollowed out in 
the lamellar partitions (Pl. 17, fig. 1) imto which the yolk cells have consolidated 
themselves. Now if segmentation obtains in a part of the yascular area, and is 
still apparently progressing externally, it is at least reasonable to expect to find 
it operating eventually wherever that area may exist; especially as the latter bears 
with it the identical uniform arrangement and modifications of yolk cells which 
are found within the circle of its primary development. So confident are we of 
the soundness of this theory, that we look earnestly forward to another breeding 
season for an opportunity to demonstrate it m an indisputable series of ocular 
proofs. 
SECTION =MAL. 
THE WHOLE EGG IS THE EMBRYO. 
Since we have shown in former pages, that the embryonic disc, and its exten- 
sion, the germinal layer, are formed by the origmal apposition of yolk-cell meso- 
blasts minutely subdivided, and that these yolk cells are all the same through 
the whole yolk mass from centre to surface, even to the very walls of the 
superficial Purkinjean vesicle ; and, moreover, since it is proved that segmentation 
obtains beyond the embryonic disc, and very probably all over and throughout the 
whole yolk, it is evident, that, in the ege of the Testudmata at least, the region 
around the Purkinjean vesicle cannot be separated from the more exterior or infe- 
rior mass which constitutes the greater bulk of the vitelline substance, and that the 
last cannot be homologous to the contents of the Graafian follicle,” which bears no 
part whatever in the formation of the embryo, but is totally exterior to the mam- 
malian egg. Again, as will be shown hereafter, that portion of the yolk which is 
originally excluded from the primary circumscription of the outlines of the embry- 
onic dise cannot be separated from the animal as an appendage? for it very soon 
1 Meckel von Hemsbach (Zeitschrift fiir Wissen- 
schaft. Zool.) and Thompson (Cyclop. Anat., London, 
1854, article “Ovum,” page 77) deny that the whole 
yolk mass of the Birds and of the scaly Reptiles 
The first of 
these writers compares the Purkinjean vesicle alone to 
corresponds to the egg of Mammals. 
the mammalian ovum, and the yolk surrounding this 
vesicle to the corpus luteum ; whilst the latter author 
includes the granular cicatricula, along with the vesi- 
cle imbedded in it, as homologous to the mammalian 
egg, and the yellow yolk to the tunica granulosa of 
the Graafian follicle. With this latter view Dr. Mar- 
tin Barry (Phil. Trans., London, 1839, p. 3809, note, 
and 370) is strongly inclined to coincide. 
2 It has been customary heretofore, among most 
authors, to designate the yolk sac as a reservoir of 
