16 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
mm. by 9-10 mm. The oviducal walls undergo modification ; the incu- 
batory chamber becomes connected with the surface of the developing 
embryo. All the surface of the latter is covered by the allantois and 
the yolk-sac, as if by two hemispherical caps ; the former predomi- 
nates increasingly over the latter. The serous membrane unites with 
both, forming an allanto-chorion and an omphalo-chorion. The allanto- 
chorion forms a placental union with the oviducal wall, and so — but 
only to a slight degree — does the omphalo-chorion. At the inferior 
pole of the embryo, there is formed at a certain stage a remarkable con- 
nection between the extra-embryonic ectoderm and the vitelline endoderm, 
but this disappears when the omphalo-chorion is formed. In the oviducts, 
after the birth of the young, there are slight retrogressive histological 
changes. The relations between the walls of the incubatory chambers 
and the surface of the embryo are in many ways comparable to the 
conditions of gestation in placental mammals. 
Formation and Fate of Primitive Streak.* — Dr. A. Robinson and 
Mr. R. Assheton have made a series of observations on the mechanism 
and germinal layers of Bana temporaria, with the object of settling 
some of the much disputed questions as to the formation and fate of the 
primitive streak. They come to the conclusion that the blastopore is a 
deficiency in the posterior wall of the archenteron ; that this archenteron 
in the Anura is not formed by invagination, but by a process of splitting 
among the yolk- cells, very similar to that described in the Axolotl. 
The situation of this archenteric cavity is first defined by the deposition 
of pigment in the adjacent margins of a double row of yolk-cells. No 
portion of the archenteric wall is formed by invaginated epiblast, for the 
archenteron is at first surrounded by large yolk-cells, which eventually 
give rise to the definite hypoblast. The ventral lip of the blastopore 
indicates the posterior end of the primitive ventral wall of the archenteron. 
There is no ventral and forward extension of the archenteron in front 
of and below the ventral lip of the blastopore by the production of a 
diverticulum in that situation. The ventral wall of the archenteron, in 
front of the ventral lip of the blastopore, is completed by the extension 
of the anterior end of the cavity, and by the withdrawal and modification 
of the cells of the yolk-plug. 
The blastopore, or so-called anus of Rusconi, is closed by the con- 
crescence of its lower lips ; this produces a median streak characterized 
by the fusion of layers, along the surface of which lies a groove ; this 
groove stretches from the ventral lip of the remainder of the blastopore 
as far as its original extension. The authors propose to call these 
structures the primitive streak and the primitive groove. Had the con- 
crescence continued a little further, so that the whole blastopore had 
been obliterated by the fusion of its lips, the primitive streak would 
have commenced at the posterior end of the notochord, as in the chick. 
It seems, therefore, clear that the homologue of the primitive streak of the 
chick is, in the Frog, the whole of the blastoporic lip, whether fused or not. 
The authors agree with Erlanger in regarding the anus of the frog 
as a reopening of a temporarily closed portion of the original blastopore, 
and not as a new perforation. They consider that the neurenteric canal, 
* Quart. Journ. Micr. Sci., xxsii. (1891) pp. 151-504(2 pis.). 
