1891.]" Embryology. 61 
are found as evaginations or invaginations of thickened ectoderm. 
Rathke, Ayers, and Graber believe these rudimentary appendages 
represent embryonic gills, Wheeler objects to this interpretation, as 
the cells of the pleuropodia are large, swollen, vacuolated structures, 
and would prevent any great interchange of gases between the blood 
and the air. Nor could these pleuropodia be sense organs, as Patten 
and Cholodkovsky believed, for no one has discovered even the trace 
of a nervous system running into them. The author is led to the 
belief that the organs must be large ductless glands, which were func- 
tional in ancestral insects. It seems probable that the pleuropodia rep- 
resent appendages homologous with the ‘thoracic legs, and may 
have been, at a remote time, ambulatory appendages, and subsequently 
converted into functional glands in the ancestors immediately pre- 
ceding living insects. 
Are the Arthropods the Ancestors of the Vertebrates ?— 
Two papers of a speculative nature have appeared side by side in a recent 
number (Aug., 1890) of the Quarterly Journal of Microscopical Science. 
The contrast between these cannot be without its value. Dr. William 
Patten, of the University of North Dakota, Grand Forks, U. S. A., 
makes the first attempt to establish “the origin of vertebrates from 
Arachnids.” The other production comes from Cambridge, England, 
and is advanced by W. H. Gaskell, M.D., F.R.S., to establish “‘ the 
origin of vertebrates from a Crustacean-like ancestor.” It is difficult 
to bring one’s self into the proper mental condition to treat these 
" Matters seriously ; but that the authors, especially Gaskell, are in dead 
earnest there can be no doubt. 
Patten seems to have been led to his hypothesis in studying the 
anatomy and embryology of Scorpions and Limulus; while Gaskell, 
if we remember aright, was started towards his present goal by the 
resemblance between fumors in the spinal cord and in the digestive 
tract. 
A detailed review of these papers is impossible here, and the barest 
Outline must suffice. Briefly, then, Patten inverts a Limulus, or a 
Scorpion, or anything similar, and proceeds to compare the principal 
organs of his up-side-down beast with a vertebrate right-side-up, prov- 
ing the identity of the structures! The most important comparison 
is between the nervous systems of the two groups. The three brain 
vesicles of the vertebrate find their homologue in the supracesophageal 
and fused thoracic ganglia of the scorpion. The cranial nerves of the 
vertebrate are the results of the first hirteen neuromeres of the Arach- 
