1885.] Embryology. 411 
EMBRYOLOGY.! 
ON THE POSITION OF THE YOLK-BLASTOPORE AS DETERMINED BY 
THE SIZE OF THE VITELLUS.—This principle, which I have to some 
extent elaborated elsewhere (Cont. Embryog. Oss. Fishes, p. 
114), in so far as it applies to the ova of bony fishes of different 
species, differing greatly in the dimensions of the vitellus, may be 
expanded so as to throw some additional light upon the growth 
and closure of the blastoderm of other groups of Vertebrata. In 
the paper cited I have shown that the position of the point of 
closure of the blastoderm in relation to the original position of 
the germinal disk in Teleostei is to a large extent determined by 
the size of the vitellus, and consequently also stands in an inti- 
mate relation to the variation of the area of the vitelline surface 
over which the blastodermic membrane must grow, that is to say, 
with the increase of the superficial area of the vitelline globe 
upon which the germinal disk is superimposed, and over which it 
spreads as the blastoderm, the position of the yolk blastopore 
must vary. 
A yolk blastopore is met with only in such forms of ova in 
which there is a distinct, unsegmented or partially segmented 
vitellus developed. As a rule, it does not coincide with the posi- 
tion of either mouth or anus, but when such a coincidence does 
occur the yolk blastopore answers nearly or quite to the perma- 
nent anus of the Vertebrate embryo. In the Vertebrates the yolk 
blastopore is apt in most cases to close behind the position of the 
permanent anus; in large-yolked cephalopod ova it closes at the 
anterior or perhaps more properly on the ventral face of the yolk- 
sack, and seems to have no relation to either mouth or anus. 
l Edited by JoHN A. RYDER, Smithsonian Institution, Washington, D. C. 
