8 Desor on the Embryology of Nemertes. 
be recognized even when not compressed, as seen in fig. 20, | 
representing a flask on the 18th day. The crescent-like spot 
is enlarged, the ciliz are more distinct, and the embryos 
revolve more rapidly than before. 
Some days after, when compressing the yolks, I noticed the 
existence of a third zone, that was interposed between the 
two others, (fig. 21) ; so that there were now three distinct 
layers in each yolk. The external one, being quite wide, 
showed very distinctly the subdivisions of the yolk substance, 
with the clear spot in each granule; the second was the most 
transparent, and there were seen in it very small and delicate 
transparent cells; the third or interior one, being the most 
opaque, with a quite distinct granular structure similar to that 
of the external zone, but more dense. 
The embryo, remained in this state until about the 24th 
day, when I noticed a most unexpected and extraordinary 
change, which, like a ray of light, made clear to me the signi- 
fication of the different zones above alluded to, and at the 
same time revealed to me a new kind of development, hith- 
erto unknown to embryologists. On putting some flasks 
under the microscope, I was astonished to see that in one of 
them there was, besides two yolks revolving in the usual man- 
ner, a third body, that had quite a different motion, elongat- 
ing and contracting. spontaneously, sometimes advancing and 
sometimes retreating. (Fig. 22 a.) It was surrounded with 
ciliæ as the others, but bad only two zones, the external 
one being wanting, and in the place of the crescent, there 
was seen a lanceolate spot, conforming in its longest diameter 
.to that of the yolk. "There were also observed in the flask 
several irregular fragments of yolk-like substance, which I had 
not noticed before, and which, judging from their appearance, 
could be nothing else than the remains of the external zone, 
which was no longer to be seen around this embryo, whilst it 
was distinct around the others. 
How had this disengagement taken place? Was it the 
normal result of the development, or was it only accidental ? 
