152 ADAM SEDGWICK. 
Wolffian blastema applies {vide fig. 1) ; but behind this 
region the development is different. 
Here^ even before the segments are formed, it is found 
that the cell mass, which on segmentation gives rise to the 
intermediate cell mass, is distinctly separate from the thick 
epithelium of the body-cavity, but attached to the cell mass, 
which will give rise to the protovertebra (fig. 6, taken 
from a chick with twenty-six protovertebrse behind the last 
protovertebra) . 
And this separation is apparently retained through later 
development. The cell mass {i c m' ^ fig. 6), in the next stage 
(fig. 7, taken from twenty-ninth segment of a chick with 
twenty-nine protovertebree) is obviously the Wolffian blas- 
tema which, in a still later stage (fig. 10, from the twenty- 
ninth segment of a chick with thirty-four), gives rise to the 
commencing Wolffian tubule. 
The same fact may be seen by comparing figs. 8 and 9, 
fig. 8 being taken from the twenty-fourth segment of a chick 
with twenty-six, and fig. 9 from the twenty-fourth segment 
of a chick with twenty-nine. 
It will be observed, by inspection of figs. 6, 7, 8, that the 
peritoneal epithelium which adjoins the Wolffian blastema is 
thick, as it is elsewhere ; while later (figs. 9, 10) it is in 
the same spot thin, as it is, or will be, in most other parts 
of the body-cavity. ' 
No doubt this thick epithelium does, in the process of 
becoming thin, bud off cells, which travel inwards, and some 
of which may help to form the definite Wolffian blastema 
(figs. 9, 10). But this process takes place everywhere. In 
fact, at an early stage of development almost all the meso- 
blast cells are represented by the thick lining of the body- 
cavity ; and it is by a process of growth inwards of cells from 
this that most of the connective tissue, &c., of the wall of the 
body and gut is derived ; and, therefore, from the analogy of 
the fate of the cells growing out from the body-cavity wall 
in other places, we might fairly assume that those growing 
out from that particular part adjoining the intermediate cell 
mass or Wolffian blastema give rise, not to the Wolffian 
tubules, but to the connective tissue and blood-vessels of the 
Wolffian body. This is rendered highly probable from a 
consideration of some observations I have made on the de- 
velopment of the segmental tubes in Elasmobranchs, which 
point to the conclusion that the epithelial lining is derived 
from the cells of the intermediate cell mass. However that 
may be, of one fact there can be no doubt, viz. the cells from 
which the Wolffian tubules develop are not derived from 
