ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
693 
Theory of the Placenta.* — Prof. C. S. Minot sums up his theory of 
the placenta of rodents as follows : — The egg fixes itself by the thickened 
ectoderm of the area placentalis, the ectoderm coalescing with the 
uterine epithelium ; the maternal epithelium, including the glands, dis- 
appears by degeneration and absorption, though deeply-situated remnants 
of the glands may remain ; the maternal capillaries of the submucosa 
become much enlarged, and their epithelium degenerates ; the connec- 
tive-tissue cells of the same layer are modified into decidual cells, and in 
the rabbit, in part into glycogenic cells ; foetal villi grow in place of 
the glands ; they branch and increase, and penetrate the maternal tissue 
until there is little more than room left for the maternal blood-courses 
in the intervillous spaces. 
Inversion of the Germinal Layers in Rodents.}* — Dr. T. Biekringer 
discusses the problems connected with this curious process discovered by 
Bischoff in the development of the guinea-pig, investigated by Kupffer 
and Selenka in the field-mouse ( Arvicola arvalis ), by Fraser in the rat, 
by Biehringer in Arvicola ( Hypudseus ) ampliibius , known also in the 
mouse, and probably occurring in Dasyprocta. The author shows that 
the conditions are simplest in Arvicola , next in the domestic mouse, then 
in the rat, most divergent in the guinea-pig, and describes each of these 
in turn. He explains how the inversion occurs, indicates clearly how 
the divergence supports, and does not weaken, the morphological con- 
stancy of the germinal layers, and shows what questions still remain 
unanswered, but the complicated facts of the case we must leave untold. 
Development of Superior Incisors and Canines of Sheep.}: — Miss 
F. Mayo states that at a certain stage in the development of the sheep, 
the dental lamina exists throughout the canine and incisor regions of 
the upper jaw. Its anterior portion, which is the last to develope and 
the first to abort, does not attain so prominent a condition as its lateral 
portion. After advancing in development for a time, it retrogrades, and 
finally disappears. In the canine region the dental lamina gives rise to 
an enamel germ, which never reaches a stage of functional activity ; its 
central cells are not transformed into a stellate reticulum, and the 
Malpighian layer never produces enamel, while in later stages both 
disappear. In this region there is no trace of a dentine germ. In the 
region of the incisors the evidences, even of the beginnings of tooth- 
development, have almost disappeared. There is every reason for 
supposing that the disappearance of the teeth has been a progressive 
process, beginning with the middle incisors and gradually affecting the 
teeth set further back. 
Blastopore of Anurous Amphibia.§ — Herr R. von Erlanger gives 
an account of his own observations on the fate of the blastopore, and 
comes to the conclusion that when his results are compared with those 
of Schanz and Morgan, we are justified in believing that the anus arises 
from the most ventral part of the blastopore, while the most dorsal forms 
the neuropore and neurenteric canal. In the Anura, that part of the 
blastopore from which the anus arises becomes temporarily closed, and 
* Biol. Centralbl., x. (1890) pp. 114-22. f T. c., pp. 403-14. 
X Bull. Mus. Comp. Zool., xiii. (1890) pp. 247-58 (2 pis.). 
§ Zool, Jahrb., iv. (1890) pp. 238-56 (2 pis.). 
