1895.] Embryology. 188 
together around the mouth, and those in the abdominal region flatten 
and become pleopods; while the thoracic limbs become the longest 
appendages of the body, with the exception of the second antenne. 
At first there is no external segmentation of the body, but gradu- 
ally a head region is marked off from the thorax by a groove. Pos- 
teriorly, the thorax and abdomen are also pretty clearly marked, 
the latter being of smaller segments. The grooves between the 
segments appear first ventrally and grow up towards the dorsal 
surface. Later dorsal grooves appear and grow down to meet the 
ventral ones. Finally the abdominal, and the two or three most ante- 
rior thoracic segments, are completely marked off by circular grooves. 
The thorax is the last region to become segmented. Its mid-dorsal por- 
tion is raised into a peculiar hump, which is very prominent for a 
time, but gradually disappears as the development proceeds to a 
finish. 
The proctodeum and stomodeum have gradually approached and 
met in the thoracic region as described, and finally the point of union 
breaks through, and the digestive tract is continuous in its whole 
length. In the first period the enteric vesicle had become almost en- 
tirely closed in around a mass of yolk in the centre of the embryo; 
now it is entirely closed and becomes greatly changed. A deep groove 
pushes in along its ventral surface from in front backward. It 
deepens towards the dorsal surface, and finally meets a groove from 
above. They split the enteric vesicle inio two halves which are how- 
ever united anteriorly by an unsplit portion. This anterior stem, bearing 
the two enteric lobes behind, (later on the two primary enteric lobes 
split into two secondary each) lies just at the point where the proc- 
todeum and stomodeum meet, and where their cavities become continu- 
ous it opens into the alimentary canal thus formed. As M. Roule says: 
“Such a disposition is found among all crustacea, with the constant 
relation of the enteric vesicle with its lobes attached to the digestive 
canal in the zone of union of the anterior and posterior intestines.” 
s Considering the entire digestive tract then, the whole system, canal 
and annexes, originates from three rudiments which are at first inde- 
pendent of each other and later joined into a single system. Two of 
these, the stomodeum and proctodeum, arise from the ectoderm, while 
the third comes from the entodeum. A like structure and like triple 
origin is to be found in the other Arthropods, but with an important 
difference: the proctodeum does not extend so far into the body as that 
of the Crustacea; its anterior end remains some distance behind the 
enteric vesicle; it unites with the enteric vesicle at its posterior end, 
