1 890.] Concrescence Theory of the Vertebrate Embryo. 715 
Concrescence involves necessarily the gradual recession of the 
blastopore; in most vertebrates the blastoporic canal merges at 
its front end into the main archenteric cavity, but in mammals, if 
the homologies I have drawn above be correct, the canal persists 
for a considerable period as the “chorda-canal” of German 
writers. 
The blastopore is not homologous with the gastrula mouth, 
but is merely a small portion thereof; in front of it the gastrula 
mouth is closed by concrescence ; while concrescence is going 
on there will be a part of the gastrula mouth open behind the 
blastopore ; when concrescence is completed the blastopore is at 
the end of the elongated gastrula mouth, the lips of which are 
united throughout the remainder of their length. The blastopore 
is not a fixed point, being merely the opening of the closed 
archenteron, and as by concrescence the archenteron is elongated, 
in precisely the same measure the blastopore travels backward, 
AUTHORITIES CITED. 
I. BENEDEN, Ep. VAN. La maturation de l'oeuf, ia fécondation et les 
premières phases du développement embryonnaire des mammifères d'après 
vin recherches faites chez le Lapin. Journ. Zool., V., 10-56, 1876. 
——. Recherches sur l' embryologie des mammifères. La formation 
ds uiet chez le Lapin. Arch Biol., I., 136-224. Pls. IV.—VI., 1880. 
3- . Untersuchungen über die Blätterbildung den Chordakanal und 
die Gastrulation bei den Säugetieren. Anat. Anz., III., 709-714, 1888. 
4. BENEDEN, ED. JULIN, CHARLES. Recherches sur la formation des 
annexes foetales chez les mammifères (Lapin et Cheiroptéra). Arch. Biol., 
V., fasc. 3, pp. 369-434. Pls. XX.-XXIV., 1884 
5. BIEHRINGER, JOACHIM. Ueber die ing der Keimblätter bei 
der Scheermaus (Arvicola amphibius Desm,). Arch. Anat. Physiol. Anat. 
Abth., 279-286. Taf. XVII., 1888. 
6. BiscHorr, THEO. L. W.  Entwickelungsgeschichte des Kanin- 
cheneies Braunschweig, 4°, pp. X., 154. 16 Taf., 1842. 
Entwicklungsgeschichte des Hunde-Eies. Braunschweig, 134 
PP-, LER pr o 1845. 
Entwicklungsgeschichte des Meerschweinchens. Giessen, 4°, 
pp. 2 3 Taf. vili. 1852. 
9. Neue Beobachtungen zur Entwicklungsgechichte des Meer- 
schweinchen. Abth. Bayr. Akad, cl.2. X., 115-166. Taf. VIL-X., 1870. 
