1885.] Embryology. 817 
envelope and no outer shell or additional membranous covering. 
Cleavage total and equal, as a result of which a blastula is directly 
developed. The invaginated half of the blastula leads directly 
to the formation of the intestine. No actinotrichia or embryonic 
fin-rays developed. These features seem to distinguish the Lep- 
tocardii, or Haplocyemate forms from the succeeding ones. 
II. The second great subdivision of the Chordata I will call 
the Epicyemate subphylum. In this series the embryo never 
becomes bodily invaginated or so pushed into the blastodermic 
vesicle as to become invested by up-growing folds of the ecto- 
blast, but remains attached to the yolk bya stalk or is sessile 
upon and external to the latter, which may or may not form an 
integral part of the intestine. This group also embraces forms 
in which no part of the ectoblast is cast off when the embryo is 
set free. There is no amnion nor any functional allantois devel- 
oped, The vessels of the yolk-sack, when developed, are respi- 
ratory in function. e primitive streak is either comparatively 
short or wanting. This subdivision answers very nearly to the 
Ichthyopsida of authors. 
1. The lowest division of the Epicyemate series may be called 
the Ichthyoidea, and brings the Amphibia and Marsipobranchii 
together. In them the ovum is provided with a zona radiata ; 
there is no outer albuminous envelope ; the zona is often elastic, 
and the cavity it encloses may increase with the progress of 
development, and may be covered externally with an adhesive or 
gelatinous investment. The cleavage is unequal ; no true blasto- 
disk is formed ; the germinal and nutritive poles are, however, 
developing intestine. Amphibia. 
6, The second subseries of the Ichthyoidea are op phous, 
that is to say, the yolk-cells while they enter into the formation 
of the ventral portion of the intestinal as in a, they are ina 
posterior position; the embryonic axis is formed from ; d 
orwards, so as to appear as if it grew out from the posteriorly- 
placed mass of yolk-cells. Petromyzon. - 
Much may be said in favor of affiliating the marsipobranchs 
with the amphibians, but whether the development of Myxine 
will countenance the foregoing arrangement remains to be 
rned. It is, however, very certain that the general plan of 
development of the two preceding series approximates that of 
Branchiostoma far more closely than the more specialized devel- 
opment of the Ichthyes. 
lS RES 1 
