138 THE VOYAGE OF H.M.S. CHALLENGER. 
to study the formation of the germ-layers, nor the modifications which they undergo 
during development. Whether all the cells of the embryo in this stage are derived 
exclusively from the blastoderm, or whether they are also partly due to the deutoplasm is 
a question which it is impossible to answer from the section before me. Dorsally the 
greater part of the embryo is covered by a single row of flattened cells (the original 
blastoderm cells), ventrally a plate is clearly distinguished much thicker than the blasto- 
derm, and doubtless formed of cells more than one row deep. Unfortunately, however, 
the limits of these cells were quite gone; I therefore could not distinguish either their 
number or arrangement, but I believe the evidence is great that in the inner layer of this 
plate the original mesoblast is to be seen. In this stage rudiments of the appendages 
are distinctly formed ; and I consider it a very characteristic feature in the development 
of the Pyenogonids, that the food-yolk penetrates into these appendages. In the section 
here figured, however, that part of the food-yolk which penetrates the leg, is not in direct 
connection with the central food-yolk mass ; but this is caused by the circumstance, that 
the section does not pass exactly vertically through the embryo, but goes a little obliquely 
from above backwards to the ventral side. 
The blastoderm shows to a considerable extent in the stage I have figured a double cell- 
layer dorsally in the middle, and even a small lumen is observed between these two. 
Small cells or nuclei seem to be present in this lumen, and the whole arrangement made 
me think it possible that I had an early stage in the development of the heart before 
me. The broad and flattened condition of the heart in the adult animal of Nymphon 
is not opposed to this suggestion ; yet it is difficult to understand why a heart should be 
developed before there seems to be any question of an intestinal tract. 
About the same stage is also figured in figs. 9 and 10. At the ventral side the 
first pair of appendages (the foot-jaws), three pairs of legs, between the foot-jaws 
the proboscis, and the caudal protuberance, are easily distinguished. The second 
and third pair of cephalic appendages show in this species a remarkable retardation 
in their appearance, visible in the stage in which the first and second pair of true 
legs are already two-jointed and bent inwards so as to meet in the middle of the 
ventral surface, and in which the third pair is longer, yet bent inwards and forwards. 
In this same stage the third cephalic appendage is not yet distinguishable, and the 
second pair only shows a small protuberance at the base of the foot-jaws. An equatorial 
section of an embryo in this same stage is figured in fig. 11. Between the foot-jaws (a) 
and the first true leg (b) two small protuberances are distinguished, the first of which (c)_ 
is larger than the second (d), which in this stage is observed only interiorly. The 
section is also remarkable for the distinctness with which the nerve ganglia are seen. 
There is good reason to consider this arrangement characteristic for the species 
Nymphon brevicaudatum, Miers. Other species of Nymphon, of course, may show 
the same; so far as I could ascertain it is not the rule, for neither NMymphon 
