128 THE AMERICAN NATURALIST, [Vor. XXXIV. 
hollow strands may in certain cases connect with the lumen of 
the central nerve tube. These hollow strands or ** roots of the 
Ptychoderide,”’ Willey says, “are genetically related to the epi- 
physial complex of Craniota; in the crucial nuchal region of the 
Enteropneusta are, therefore, to be found not the actual but 
the nearest possible approximation to the actual primordia of 
the . . . epiphysis cerebri of Craniota.” Of course this may be 
true, yet it seems questionable if it be “ profitable for doctrine." 
Willey's fifth proposition he states as follows: “Just as the 
medullary tube of the collar is admittedly an invaginated por- 
tion of the dorsal nerve trunk,! so the medullary folds which 
arise and fuse to form the medullary tube are to be regarded as 
specializations of the anterior portion of the pleural folds which 
are retained in the Ptychoderidz as the genital pleura.” * The 
genital folds of Enteropneusta, the atrial folds of Amphioxus, 
and the medullary folds of Vertebrata belong to the system of 
pleural folds of the body wall, and are differentiated from a 
common primordium." In this connection it may be well to 
remember that Amphioxus has both medullary folds and atrial 
folds, and that there is no apparent relation between them. 
Willey's sixth proposition refers to a recession of the “ pos- 
terior neuropore " until it reaches and becomes associated with 
the blastopore (** primitive anus ”) to form the neurenteric canal. 
I do not, however, clearly understand the terms he uses. 
Dr. Willey next discusses briefly the different regions of 
pseudo-chondroid tissue, z.e., ‘stomochord, pygochord, and 
pleurochords." 
Willey points out that the tongue bars of the gill region of 
the Enteropneusta “are not (ontogenetically) secondary, as they 
are in Amphioxus," and that * by their development, size, and 
1 Cf. Morgan (Journal of Morphology, vol. ix, 1894, p. 74). “ We seein Balano- 
glossus that the ixvaginated dorsal nerve cord can correspond only to the anterior 
end of the nerve cord of Amphioxus, and that the superficial dorsal nerve path, 
stretching through the gill region, thence to the end of the body, must be the 
homologue of the remainder of the nerve cord of Amphioxus." 
