SUMMARY OF CURRENT RESEARCHES 
RELATING TO 
ZOOLOGY AND BOTANY 
(PRINCIPALLY INVERTEBRATA AND CRYrTOGAMIA), 
MICROSCOPY, Etc. 
Including Original Communications from Fellows and Others .* 
ZOOLOGY. 
VERTEBRATA. 
a. Embryolog-y.f 
Span of Gestation and Cause of Birth. J- — Dr. J. Beard lias given 
this title to another of his suggestive papers. It should have been 
reported on sooner, but the author gives one much to ponder over. 
In the early days of Mammalian evolution, before an allantoic 
placenta had arisen, the birth period and the critical period coincided, 
as is still seen in many Marsupials. Then, too, the ovulation period 
must have been almost equal to — really a little longer than — the critical 
period. A coming ovulation, a reflex message from ovary to uterus, 
was the direct cause of birth. 
The evolution of an allantoic placenta made it possible for birth to 
be postponed to a later time, for the new structure provided for the 
nutrition of the foetus beyond the critical period or, more technically, 
beyond the single “ critical unit.” Then, too, the ovulation period 
was extended to embrace two critical units instead of one, as still seen 
in mouse, rabbit, &c. This marked the passage from the Metatherian 
to the Eutherian status. 
A gestation of two units, although an advance on a gestation of one 
unit only, did not yield the advantages to be gained by still further 
prolongation to include three or more critical units. This further pro- 
longation was acquired, but still preserved the correspondence between 
the length of gestation and a certain number of critical and ovulation 
units. Illustrations are given of cases up to eight critical units. 
* The Society are not intended to be denoted by the editorial “ we,” and they do 
not hold themselves responsible for the views of the authors of the papers noted, 
nor for any claim to novelty or otherwise made by them. The object of this part of 
the Journal is to present a summary of the papers as actually published , and to 
describe and illustrate Instruments, Apparatus, &c., which are either new or have 
not been previously described in this country. 
f This section includes not only papers relating to Embryology properly so called, 
but also those dealing with Evolution, Development, and Reproduction, and allied 
subjects. 
X ‘ The Span of Gestation and the Cause of Birth ; a Study of the Critical 
Period and its Effects in Mammalia,’ Jena, 1897, 8vo, ix. and 132 pp. 
