REVIEW OF SPENGEL’S MONOGRAPH ON BALANOGLOSSUS. 407 
tion has nothing to do with its formation, but only with its 
removal from the surface, since nervous matter is only found 
on its ventral and lateral surfaces. In adducing such con- 
‘siderations as in any way telling against its similarity to the 
nerve-cord of the Chordata, Professor Spengel betrays his hazy 
conceptions of Vertebrate embryology. The nerve-cord of 
Amphioxus, like that of Balanoglossus, appears as a 
median differentiated strip of ectoderm before any 
invagination commences; and so marked features do the 
ventral position of the white matter and the presence of 
numbers of non-nervous cells in the cord form in the develop- 
ing lower Vertebrata, that one investigator has put forward 
the startling suggestion that the nerve-cord of Vertebrates 
represents the alimentary canal of Arthropoda with which the 
ventral nerve-cords have fused. That the collar-cord is really 
the central nervous system appears from Professor Spengel’s 
account of its anatomy ; it is no argument against this to say, 
as he does, that though ganglion-cells are more abundant in it, 
yet they occur sparsely in the trunk and dorsal and ventral 
cords. A similar mode of reasoning would prove that the 
nerve-ring and radial cords of Echinoderms are not central 
organs. 
Professor Spengel’s objections to regarding the organ which 
Bateson calls the notochord as homologous to the notochord 
of Vertebrata are of two kinds, histological and topographical. 
We shall deal with the former first. 
Bateson stated that the head of the notochord was solid, 
and that its tissue was remarkably like that of the notochord 
of a young Elasmobranch. Spengel maintains that there is a 
narrow lumen throughout, though sometimes interrupted ; 
and though there is a superficial resemblance in transverse 
section to notochordal tissue, yet it is only superficial, for the 
cells are epithelial cells, modified by the development of huge 
vacuoles, whereas he says true notochordal tissue consists of 
spherical cells more or less flattened against each 
other with vacuoles, and in the chinks between the spheres a 
gelatinous substance, This kind of tissue he thinks it im- 
