422 MALL. [VoL. XII. 
The aortae do not unite, but each sends a number of segmen- 
tal branches to the umbilical vesicle along the tail end of the 
embryo. These are, of course, temporary; they maybe 
called collectively the omphalomesenteric arteries. As the 
permanent omphalomesenteric artery arises more aboral than 
any of these, it is easy to understand that most of them must 
degenerate. 
The sections show that there are fourteen muscle plates, all 
of which are hollow and do not in any way communicate with 
the body cavity in general. Kollman, who described an embryo 
of this same age, numbers them from before backward, but I 
think that they can be designated more definitely. Froriep? 
showed that in all amniotic vertebrates there were a number of 
muscle plates and dorsal ganglia formed in the occipital 
region, and studied their fate in the chick and in the cow’s 
embryo. Platt? has also followed the order of the origin of the 
muscle plate in the chick, and found that the first division of 
the mesoderm was between the third and fourth occipital plates. 
The first three or four of these segments communicate in the 
chick, according to Dexter,? with the coelom, and Bonnet ‘ has 
found also that the same is true in the sheep. Bonnet’s fig- 
ures (compare his Plate IV) show that a sheep’s embryo of the 
same stage as embryo XII has muscle plates much more sharply 
outlined than the human. In order to locate the muscle plates 
more definitely I have made every effort to count the spinal 
ganglia in embryo XII, but with no definite result. It is 
impossible for me to define the spinal ganglia, as often they 
are represented by a few cells only, then again as a band of 
cells they extend over several segments. The same is true in 
the occipital region. Had I been able to number them defi- 
nitely it would still have been impossible to number the muscle 
plates from them, for His® has shown that there is an occipital 
ganglion in the human embryo as well as in the lower animals. 
The fact that the muscle plates reach to the otic vesicle in 
1 Froriep: His’s Archiv, 1883 and 1886. 
? Platt: Bulletin of the Museum of Comparative Zodlogy, vol. XVII. 
3 Dexter: Anatom. Anz., 1890. 
4 Bonnet: His’s Archiv, 1880. 
5 His: Abhandl. d. sich. Gesellsch. d. Wiss., Bd. XXIV. 
