92 Walter E. Dandy. 
first pair of somites, which according to experiments in the chick 
by Marion Hubbard and Patterson, is established first in time and 
position, but develops slowly and maintains its connection with the 
anterior paraxial mesoderm, thus causing the formation of somites 
to take place only posteriorly. 
Celom. In his article on the eclom for the new Keibel-Mall 
Embryology (now in the press), Professor Mall used this embryo 
as the second stage of human eccelomic development. Graf Spee’s 
Gle 1.54 mm. is the first human embryo to show any sign of eccelom 
formation, this being a very small bilateral slit in the mesoderm of 
the anterior portion of the embryo—the beginning of the pericardial 
colom. This slit is said to communicate externally by a very small 
channel. The next stage, represented by this embryo, shows a very 
large and well developed single, united pericardial cavity. (Figs. 
1 and 11.) 
The posterior continuation of the pericardial cavity on each side 
(Figs. 11 and 15) probably represents the pleural ccelom, which 
passes insensibly into the peritoneal eeelom, now forming from 
multiple foci. A glance at Fig. 11 will show the extreme irregularity 
and entire absence of any metameric arrangement in the formation 
of the peritoneal eclom. Numerous irregular pockets of varying 
size dip into the mesoderm from the extraceelom. This communica- 
tion with the exterior may be either primary or secondary. The 
diagram of the reconstructed ceelom, however, would seem to indi- 
cate the former might be the case, because posteriorly a rather long 
continuous slit is present in the mesoderm, the pockets of which are 
shorter, much wider at the external opening, and diminish in size 
internally. There are, however, several small independent cavities, 
having no visible connection with each other or with the extraccelom, 
which strongly suggest an independent formation by internal meso- 
dermal cleavage, beginning in multiple small foci, which may later 
connect with the other cavities and thus indirectly through them to the 
extracelom. Judging from the structure of this specimen alone it 
seems probable that the result may be a combination of both processes 
which are in reality ultimately one and the same process differing 
only in position. This irregular formation has been described by 
