18 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
and pliylogenetically. The boundary between the two regions is marked 
by the auditory vesicle, wdiich is reckoned, however, with the anterior 
portion. The mesoderm of the anterior head may be divided into 
several portions, but these are not comparable to vertebral segments 
( Urwirhel) either in origin or in differentiation. In the anterior head 
there are (apart from olfactory and optic) two primary nerves, the 
Trigeminus and the Acusticofacialis. The nerves of the eye-muscles 
are perhaps to be derived from the Trigeminus, and the muscles them- 
selves perhaps from the visceral musculature associated with the first 
arch. The primary nerves of the posterior head are the Glossopha- 
ryn gens and the Vagus; the Hypoglossus also arises from the ventral 
roots of this region. A portion of the Vagus may also attain inde- 
pendence as the Accessorius. Homologues of dorsal branches are not 
to be souglit for, since they arise in the trunk at a late stage, apparently 
in connection with the splitting of the originally single lateral muscular 
mass into dorsal and ventral regions. The unsegmented mesoderm of 
the anterior head of Craniota is compared to the process of the first verte- 
bral segment in Ainphioxus, as described by Hatschek. 
Placenta of Rodentia.^ — M. M. Duval commences his account of 
his own observations with a description of the placenta of the Eabbit. 
For the terms plasmodiblast or cytoblast, suggested by Van Beneden for 
the part formed from the ectoderm of the egg, he proposes the vox 
liijbrida ectoplaconta. On the seventh day of gestation, that is just 
before the fixation of the ovum to the mucous membrane, the latter 
exhibits the modifications by which the maternal placenta is distin- 
guislied; these are, macroscopically, the formation of the two cotyle- 
donary projections, which are separated by a wide and deep inter- 
cotyledonary groove ; the histological appearances are the conversion of 
the uterine epithelium into a homogeneous layer, and the development 
of the capillaries of the mucous membrane. 
The development of the foetal jwt of the placenta commences at 
the end of the seventh day with an ectodermal thickening in the form of 
ectoplacental crosses. In these there are a number of layers, the 
superficial of which form the plasmodial layer of the ectoplacenta, while 
the deeper remain formed of distinct cells. In the former the nuclei 
multiply by direct division, in the latter by karyokinesis. The former 
increases by outgrowths which make their way into the mucous mem- 
brane of tlie cotyledonary projections of the uterus ; at the end of the 
ninth day they more or less completely surround the superficial capil- 
laries of this mucous membrane. At the same time every trace of the 
epithelium of the uterus disappears at the level of the ectoplacental 
formation, and there only remain glandular caeca. 
After the ninth day the elements of the plasmodial layer of the 
ectoplacenta surround the superficial capillaries of the uterine cotyle- 
dons, and, owing to the disappearance of the endothelial wall which 
alone limited these vessels, they become reduced to mere sinuses 
liollowed out in the substance of the ectoplacenta, that is to say, to 
sinuses bounded by the ectodermal elements of the embryo and filled 
with maternal blood, 
* Journ. Anat. et Physiol., xxv. (1889) pp. 309-42 (2 pis.). 
