382 The American Naturalist. 
the blastoderm, which then adheres firmly to the shell-membrane, ht 
there is no space for the head fold to grow, except towards below.’ 
The Placenta of Rodents.—Duval has published a paper in 
Journal de l’ Anatomie? giving a clear and interesting account of 
discovery of the “‘ inversion ’’ of the germ-layers of rodents, a 
theory of the method by which this curious process has beani 
indeed, Duval believes that, although often in error, Bischoff 
the fundamental meaning of the phenomenon, and had a 
knowledge of the process than some of the investigators that 
after him. 
Primarily the KAKES of the blastodermic portion of the em 
vesicle has been the cause of the inversion of the layers. ‘This was clo 
connected with the formation of the ectodermic amnion which 
up over the embryo as the latter sinks into the vesicle. By thie 
of the amnion are there is formed a cavity lined by € 
which, owing to the Sinking of the embryo, lies, as it were, 
center of the vesicle, but still adove the embryo. This ai 
place in its simplest form in the hare, but in the other rodents i 
place by an abbreviated condition; for the amniotic (ect 
_ Space first appears as a sf/it in the thickened ectoderm above 
derm. Subsequently the cavity of the amnion is divid 
parts, an upper and a lower, by a constriction formed in t! 
This division the author believes primarily to have taken place 
early development of the allantois in order that it might 
under the upper (attached) pole of the embryo. In many i 
amnion divides into its two parts before the appearance of 
tois, and this is but a precocious process, caused in the ms 
the growth of the allantois. 
On the Morphology of the Bilateral Ciliate 
‘the Echinoderm Larvæ.!—In a previous work (Die 
2? XXVI. Anne., No, 6, 1891. 
ČR. Semon, Jenaische Zeitschrift, XXV. (N. F., XVIIL), 1890. 
