1885.] Embryology. 1117 
lower forms been very generally perforated in the median line 
anteriorly to form the permanent mouth, and posteriorly to form 
the anus. The secondary modifications which have affected this 
mode of development of the permanent openings into the enteron 
depend, apparently, in large part upon a change in the aspects of 
the body, especially in the chordata in which the permanent 
mouth and anus are both new developments and do not coincide 
with the mouth and anus of primitive Bilateralia. 
The primitively elongated mouth of the larve of Bilateralia, 
with an extended body-axis, or any derived form of the latter, or 
wherever there is formed a well-defined, unpaired median neural 
plate, or where a pair of parallel neural plates or cords are devel- 
oped, I would call the whole area thus embraced an archistome. In 
the higher forms this archistome would be coéxtensive with the 
neural groove antero-posteriorly, as far forward as the pineal 
body, and as far backward as the true secondary blastopore, and 
even beyond it, when a primitive streak was formed by the con- 
cresence of the limbs of the blastoderm behind the posterior end 
of the axis of the embryo. In other words the archistome would 
extend from the pineal body in chordate embryos along 
the whole length of the embryonic axis through its blastopore 
and on through the primitive streak to the point where the yolk- 
blastopore closed. If the archistome were, therefore, to remain 
Open, it would present the appearance of a cleft dividing the 
embryo into two symmetrical halves through the median line, and 
would extend even through the aborted portion of the lips of the 
Primitive blastopore when a very long primitive streak was de- 
veloped. It is thus rendered evident that I do not regard the 
unmodified, round gastrula-mouth, as understood by Haeckel, as 
always representing all of the blastopore in higher forms. Ac- 
cording to this view the original gastrula-mouth is in fact greatly 
elongated as a result of growth in length, in consequente of 
which bilaterality becomes established, and of which we have the 
first hint in the Actinozoa. This is further intensified by de- 
velopment from before backwards, since, without exception, the 
elongate Bilateralia differentiate the cephalic end of the body in 
advance of the caudal. In confirmation of the foregoing views I 
would refer the reader to the existing special memoirs on the de- 
velopment of the primitive grooves and blastoderms in the fishes 
and arthropods (Tracheates especially). i a 
Furthermore, the phylogeny of the mesoblastic somites is abso- 
lutely untraceable to any other source except to the gut pouches 
of a bilateral type approximating the Actinozoa, and whether the 
process has been abbreviated in arthropods or not, we are at least 
certain that in some primitive Chordata, the Teleostei, for ex- 
\ 
ample, the proof that the mesoblastic somites of the body grow 
from the concresced lips of the blastopore are so conclusive as to 
be incontestible. The way in which the mesenteron arises, and 
