BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
formed the longitudinal musculature. It would seem, therefore, that 
the muscle plate of the mesodermal somite of Amphioxus is homologous 
with the muscle cells of the Ciona tadpole. Both in Amphioxus and in 
Ciona the muscle fundament arises from cells lying lateral to the chorda 
and derived from the primary ectoderm. In Amphioxus the muscula- 
ture, like the chorda with which it is intimately associated, becomes 
(ecenogenetically ) extended far forward to the anterior end of the 
trunk region; whereas in Ciona neither musculature nor chorda extends 
farther forward than about the middle of the trunk region. 
The mesoderm lateral to the muscle plates of Amphioxus seems to be 
the homologue of the mesenchyme of Ciona. Both are derived from 
the endodermal portion of the mesoderm. | (Cf. the quotation from Lwoff, 
pages 268, 269.) 
i My conclusions differ from those of Lwoff chiefly regarding tho origin 
ofthe chorda. He considers this organ to be derived from the primary 
ectoderm in Amphioxus and the Vertebrates, whereas | regard it as 
formed in Ciona exclusively by the primary endoderm. I think that 
Lwoff has been led to include the chorda cells in the primary ectoderm 
chiefly because they are in Amphioxus (as in Ciona) smaller and clearer 
than the less rapidly eleaving endoderm cells. "These criteria I regard 
as insufficient. Only a study of the cell lineage can give in any case 
a positive answer to the question whether the chorda cells at the begin- 
ning of gastrulation lie in the outer or the inner layer of the embryo. 
That a distinction is rightly made in the case of Ascidians between 
the two kinds of mesoderm which I have recognized, viz. musculature 
and mesenchyme, is unanimously agreed to by embryologists ; but the 
fact has been heretofore overlooked that these two kinds of mesoderm are 
derived from different fundaments early distinguishable both histologically 
and topographically, and that these fundaments chould be regarded as 
derived from different primary germ layers. 
A minor point of theoretical importance is whether or not the chorda 
shall be regarded as a mesodermal organ. Lwoff does not so consider it, 
though he recognizes two facts which, it seems to me, would naturally lead 
one to that conclusion: the first, that in Amphioxus and the lower groups 
of Vertebrates the chorda is derived from a common fundament with 
what is universally regarded as mesoderm ; the second, that the chorda, 
like the undoubted mesoderm, comes to occupy a position between the 
inner and outer layers of the embryo. For these two reasons, which 
I have shown to exist also in the case of Ascidians, we must, to be con- 
sistent, regard the chorda as a mesodermal organ. 
