A Human Embryo with Seven Pairs of Somites. 91 
becomes smaller anteriorly, being represented by a very small thick- 
ening of entoderm (Fig. 3) and finally is indistiguishable. Except 
in the region of hindgut mentioned above, it, together with the ento- 
derm, is everywhere in contact with the neural groove ectoderm, usu- 
ally a line of contact being visible; in some places however a line of 
demareation is exceedingly difficult to make out. No trace of a 
notochordal eanal could be detected. 
M&SoDERM. 
Mesoderm.—Posterior to the somites is a large mass of very sim- 
ple undifferentiated mesoderm which dorsally passes insensibly 
into the primitive streak. It is largely responsible for the size and 
shape of the posterior region of the embryo. Its only differentiation 
is laterally, where it becomes somewhat more compactly arranged, 
lining the spaces which are forming the peritoneal celom. Anterior 
to the somites the paraxial mesoderm is also very simple and serves 
as a wedge of tissue between entoderm and ectoderm. The highest 
mesodermal differentiation is seen in the formation of the somites, 
coelom and heart which will now be considered. 
There are seven pairs of somites completely segmented. Each of 
the first six pairs contains a cavity, unconnected laterally with the 
eeelom as described by Keibel in the reconstruction of the Kroemer- 
Pfannenstiel embryo, Klb. The first pair is apparently the most 
highly differentiated, being the largest, having the largest cavity and 
most compact peripheral arrangement of cells and is connected later- 
ally by the most clearly defined and differentiated intermediate cell 
mass. These characteristics are less pronounced in each successive 
posterior somite until the last is the smallest, contains no cavity, 
has no cellular arrangement, and shows no other signs of differentia- 
tion. This would seem to indieate that the cavity is of secondary 
origin; Graf Spee however finds a small mesodermal slit in embryo 
Gle which he thinks is a myocele, although no somites are present. 
In addition to the completely formed somites, another pair is just 
beginning to show signs of constriction from the posterior paraxial 
mesoderm. Anteriorly there is also another pair of somites about 
half segmented with a very uniform compact mass of cells but con- 
taining no cavity. This rudimentary pair probably represents the 
