2 The American Naturalist. [January, 
(ectodermal) becoming flattened, while the entodermal are larger 
and more columnar. Next the sides of this placula-like structure 
are bent downwards, the approaching edges forming the slit-like 
blastopore, which closes behind, leaving the anterior end open as 
the mouth. In this operation the primary mesoblasts lie side by 
side at the posterior lip of the blastopore, the bands extending 
forward and eventually uniting in front of the mouth. During 
this process the ectoderm becomes thickened immediately over 
each band, but retains its character of a layer a single cell in 
thickness. At the same time, although the blastopore never 
closes, the inpushing of a stomodzum occurs. 
The next feature of interest is the formation of the middle layer 
of the germ-bands, the existence of which was first distinctly 
recognized in the Oligochætes by Dr. Wilson. According to his 
account, this first appears as a linear arrangement of the ectoderm’ 
cells, terminating behind in a larger cell. These terminal cells 
(which, like the primary mesoblasts, are called teloblasts) sink, 
together with the corresponding cell-rows, into a position be- 
tween the ectoderm and the germ-band proceeding from the meso- 
blast.. There may be either three or four of these rows on either 
side. The one towards the median line gives rise to the nervous 
system, the next two (nephric rows) to the nephridia and to the | 
inner series of setigerous glands, while the outer (not constant) 
has a problematical fate. The corresponding teloblasts are called 
neuroblast, nephroblast, and lateral teloblast. 
With the gradual elongation of the embryo these teloblasts con- 
tinue to bud off new cells, which add to the corresponding row, — 
which derive nothing from any other source. Behind, the rows a 
are but a cell in width, but anteriorly they are wider, the two 
nephric rows becoming fused. Later the teloblasts disappear, and 
the bands of the opposite sides unite from before backwards. 
Certain cells budded from the rows wander between ectoderm and 
entoderm and form “ migratory mesoderm,” while the remainder, in a 
which the ccelomic cavities appear, is the “ trunk mesoderm.” 
former is mesenchymal in character, and gives rise to larval muscles, - ) 
which are later replaced by true mesothelial muscles. The 
* See below for Bergh’s account, which differs considerably. 
