456 SUMMARY OF CURRENT RESEARCHES RELATING TO 
organisms is the result of a growth of their surface, without fresh 
increase in size ; this causes involutions of the surfaces. 
Fuller details are promised in a more extended memoir. 
The Yolk-organ of Vertebrata.* — Dr. Hs. Virchow discusses the 
yelk-organ of Reptiles and of Amphibia in comparison with that of 
Birds. In reptiles the appendages to the wall of the yolk-sac develope 
as richly as in birds, but relatively late; the primary circulation is 
homologous ; as regards the secondary arrangement of the vessels there 
are differences between the orders of reptiles as well as between reptiles 
and birds ; in both classes the yolk-sac is taken up into the body-cavity ; 
it seems likely, though not certain, that the vitelline duct in reptiles 
completely closes and disappears; Giacomini has described in Seps 
chalcides an allantoic-placenta at the proximal pole and a yolk-sac 
placenta at the distal pole — the latter, perhaps, comparable to a struc- 
ture described by Duval in the bird. As the result of his comparison, 
Virchow concludes that reptiles are, as concerns yolk-sac, in no wise 
less differentiated than birds. But a study of the development of the 
“ yolk-entoblast” leads him to believe that the yolk-organ of reptiles is 
essentially nearer that of amphibians. The nearer relationship rests on 
two facts — in the formation of typical large “yolk-cells” and in the 
occurrence of a yolk-segmentation. His description of the polymorphic 
yolk entoblasfc of reptiles is necessarily complicated. The epithelial 
“ lecithoderm ” containing cells with and without yolk is distinguished 
from the free cells which are not disposed in an epithelium. The free 
cells include (a) merocytes, rich or poor in protoplasm ; ( b ) typical large 
yolk-cells, spherical or flattened ; (c) cells without yolk, both round and 
flat ; and ( d ) very small cells without yolk. These diverse elements 
the author describes, comparing what is seen in reptiles with what is 
seen in Selachians, Amphibians, and Birds. 
Fertilization of Reptilian Ova.f — Dr. A. Oppel, in investigating 
the early stages in the development of Anguis fragilis and Tropidonotus 
matrix , finds in the germinal disc several sperm-nuclei which occur 
under small pits on the disc. At the end of the first division of the 
oosperm-nucleus the accessory sperm-nuclei are still present, and some 
of them undergo division in an irregular fashion. Their presence seems 
to be due to polyspermy, and as they occur also in Lacerta viridis , it 
seems as if this were common in Reptilian ova. Even in the stage 
with sixteen segmentation-nuclei the persistence and the division of the 
accessory nuclei may be observed, but they take no share in forming 
the embryo. 
Gastrulation in the Tortoise.:}: — Herr L. Will has studied embryos 
of Testudo lutaria , and while confirming what Clark observed in regard 
to gastrula-invagination, has made several discoveries. The archenteron 
is remarkably large. The “sickle” and the primitive plate arising 
therefrom lie originally outside the embryonic shield. All the endoderm, 
including the sickle, arises not from a proliferation of ectoderm, but from 
the inclosure of segmentation elements which were already in loco. 
* Zeitschr. f. Wiss. Zool., liii. Suppl. (1892) pp. 161-206 (1 ph). 
t Arch. f. Mikr. Anat., xxxix. (1892) pp. 215-90 (4 pis.). Anat. Anzeig., vi 
(1891) pp. 536-44 (4 figs.). J Biol. Centralbl., xii. (1892) pp. 182-92 (4 figs.). 
