BLASTOPORE, MESODERM AND METAMERIO SEGMENTATION. 17 
the cells of the middle of the primitive streak is as follows. 
About the middle of the primitive streak the ectoblastic 
elements divide very rapidly, and very soon the primitive 
groove disappears in this region (cf. PL II, figs. 2 — 6) . Coinci- 
dently with this a space appears between ecto- and endoblast 
(PL II, fig. 18, v ). Consider the fate of a single one of the 
cells of the primitive streak. This cell is destined to give rise 
by division to both ecto- and endoblast. The ectoblastic por- 
tion increases more rapidly than the endoblastic, and soon the 
latter is no longer in contact with the former, i.e. a space has 
arisen in the median ventral line. This space extends both 
anteriorly and posteriorly. The primitive groove now only 
remains as a pit at the posterior end of the embryo (fig. 4, y). 
The anterior opening of the blastopore remains open and 
becomes the mouth of the future Phoronis, the posterior pit 
is destined to undergo some very remarkable changes which 
will be described below. During these changes the original 
solid mass of cells — the posterior portion of the primitive 
streak — remains unaltered as a cord connecting the archenteron 
with the ectoderm. 
The Separation of the Mesoblast. 
Previous Observations. — Kowalewsky originally de- 
scribed the mesoblast as originating in Phoronis by delamination 
from the ectoderm (vide Plate II, fig. 13). This mistake 
arose from the ectoderm cells being darker at their base. 
Recently Metschnikoff and Foettinger have attempted to solve 
the problem of mesoderm formation. While they have both 
recognised Kowalewsky's error, they have fallen into other 
mistakes. Metschnikoff describes some mesoblastic cells already 
present in the blastula stage ; he figures four of them in his fig. 
30. In each cell Metschnikoff has drawn a nucleus. I have fre- 
quently observed this appearance of cells. It is caused, how- 
ever, by the amoeboid processes of the endoderm cells growing 
into the segmentation cavity. This is easily proved by making 
real sections. Another possible explanation of Metschnikoff's 
account may lie in the presence of certain peculiar bodies in 
VOL. XXV. NEW BER. 
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