76 BULLETIN OF THE 
two lateral elements has been developed, and the stomodzum has be- 
come faintly marked (Pl. III. fig. 16, Pl. IV. fig. 23). The four pairs 
of provisional appendages are now fully established, and the embryo has 
increased in length till the head- and tail-lobes are nearly in contact ; 
the dorsal region is, as a consequence, much reduced. Behind the 
somites which bear the provisional appendages the tail-lobe has given 
rise to at least six indistinct additional somites ; the terminal end of the 
tail is much narrowed and is becoming more pointed. The swellings 
produced by the rudimentary ganglia, at the bases of the appendages, are 
further developed, and the median ventral furrow has increased both in 
depth and in width. 
Balfour has given good figures and descriptions of the germinal layers 
during the formation of the appendages. The mesoderm is of especial 
interest at this time. Early in the protozonite stage it forms a con- 
tinuous band, about as wide as the embryo, composed of a single layer of 
cells extending the whole length of the embryonic band. About the 
time the appendages begin to appear the mesoderm splits along the 
median ventral line, thus forming two parallel bands, which remain 
united, however, in the head and tail regions. The division of the 
mesoderm into lateral halves is followed by an increase in the thickness 
of the resulting bands, each of which becomes split into a somatic and a 
splanchnic layer. It is also at this stage that the mesoderm is divided by 
transverse constructions into somites, each of which contains a central 
lumen. I am unable to determine from my specimens whether its divis- 
ion into successive blocks precedes or follows the appearance of the 
lumen. In the growth of the appendages the somatic layer of the meso- 
derm accompanies the outgrowing ectoderm, and forms a continuous 
lining to its cavity. 
During this period the ectoderm has also increased in thickness, but 
along the median ventral line it remains thinner; from this it results 
that there are two bands of thickened ectoderm corresponding to the two 
deep bands of mesoderm. The ventral median depression previously 
mentioned is at first due to the relative thinness of the ectoderm in this 
region ; it is afterwards made more conspicuous by the further separa- 
tion of the mesodermic bands. From the ectodermic bands are formed 
the nervous ganglia. They are developed first in the thoracic region in 
the form of swellings at the bases of the appendages, but by the time the 
stage represented in Fig. 8 has been reached, they have also been formed 
in the abdominal region. 
As already correctly maintained by Balfour, the segment of the cheli- 
