1885. | Embryology. 413 
with the mode in which the embryonic axis is formed during 
vertebrate development, that in the former growth in length of the 
axis would necessarily be completed before the blastoderm could 
spread over and include the yolk. Those forms of vertebrate 
embryos in which either the true or the yolk blastopore marks 
the end of the embryonic axis before the appearance of the tail 
bud might be called ze/eporous, while those in which there is no 
such coincidence, the yolk blastopore closing some distance behind 
or remotely away from the end of the embryonic axis, might be 
called, ateleporous. The first would include Amphibia, Petromyzon, 
Ganoidei, Chondrostei and Teleostei, the last, Elasmobranchs and 
Sauropsida. The ova of the two extremes of the vertebrate series 
Branchiostoma and Mammalia are yolkless, except those o 
Monotremata, which are probably ateleporous, simulating the 
Sauropsida in the general features of the development of the 
blastoderm and early phases of the embryo. 
The band of tissue from the vitelline end of the umbilical stalk 
to the edge of the blastodermic rim in Elasmobranchii, and the 
primitive streak in Sauropsida and Mammalia are probably homol- 
Ogous structures. In the first instance it is formed by the con- 
crescence of the margin of the blastoderm as it advances over the 
surface of the vitellus. In the Teleostei, Ganoidei' and Chon- 
drostei it would seem that the whole of the margin of the blasto- 
derm was used up by a process of concrescence to form the em- 
bryonic axis, whereas in the Elasmobranchii and Sauropsida there 
is a portion of the rim of the blastoderm remaining behind the 
development of the Sauropsida in the way in which it occurs in 
Ichthyopsida, it is known that the primitive streak is related pos- 
teriorly on either side to the rand-wuilst or ‘marginal thickening 
of the chick's blastoderm, a structure obviously homologous with 
the lower layer of the thickened margin of the blastoderm of the 
a relation to the b: y and int 
fishes and totally unlike that noticed in 
zonts, 
h 
be seen, differs but slightly from that of bony 
also supported by the way in which the y 
relation to the body of the embryo. 
