LEUCKART, ON PENTASTOMUM TANIOIDES. 189 
applied membrane. ‘This groove, however, only remains for 
a short time in its primitive form; it very soon deepens 
into a cleft, which penetrates into the yelk-mass towards the 
posterior pole in a diagonal direction. The yelk-segment 
which is separated by this notch, and coheres with the re- 
maining yelk, only on the dorsal surface of the egg, is the 
first indication of the tail, of which we have already stated 
above, that it is constantly folded upon itself forwards under- 
neath the abdomen whilst the embryo remains within the 
egg-coverings. 
After the yelk has thus in general assumed the morpho- 
logical conditions of the future embryonic body, it likewise 
commences to retreat at other points from the membranous 
layer with which it is coated, and to clothe itself, with a new 
cuticular covering, the future chitinous shield. The former 
vitelline membrane in this manner becomes a capsular 
covering of the embryo, in the same way as the two primi- 
tive egg-envelopes formed a long time previous; it becomes 
the innermost coat of the ovum. It is true that, on the 
dorsal process, this envelope is for a time connected with the 
embryonic body inclosed within. But this connexion is like- 
wise gradually dissolved, from the circumstance, that the 
grooving in the centre of the dorsal process above mentioned, 
deepens more and more, and finally separates the structure in 
question into two isolated parts. The upper part remains 
seated on the internal coat of the ovum, forming the “ facet” 
seen by Schubart, while the lower portion becomes the dorsal 
depression of the embryo, the margins having in the mean- 
while become continuous with its chitinous covering. After 
the separation an alteration often takes place in the relative 
position of the embryonic body and the internal egg-shell, 
and on this account, as Schubart has also remarked, we 
often find the facet placed laterally or even on the ventral 
surface of the embryo. 
Even before the separation of the future dorsal depression 
is completed from the facet of the interior embryonic cover- 
ing, the pedal tubercles, with their claws, have also made 
their appearance. They arise at a time in which the rudi- 
ment of the tail has but just been formed and the dermis still 
appears extremely thin, and in the form of two elevations at 
the sides of the anterior body. The claws are formed earlier 
than the chitinous ridges, and in respect of the latter again 
the prongs of the fork earlier than the stem. At the same 
time there arises along with the claws, at the anterior extre- 
mity of the embryonic body, a roundish depression, consti- 
