£52 THE POPULAR SCIENCE MONTHLY 
of two longitudinal folds extending above the line of insertion of the 
fore- and hind-limbs which have already made their appearance. Hence 
the carapace grows outward and over the limb-girdles which come to lie 
within the rib-like osseous supports. This ontogeny shows us clearly 
how the ancestral carapace may have been formed. Paleontology has 
not as yet confirmed this, but doubtless will do so. 
Among birds one of the most interesting features is the occurrence 
of vestigial tooth papille in the jaws of certain embryo parrots and owls 
reminiscent of Mesozoic days when birds were toothed in their adult 
state. 
Mammalian evidence is very striking in many details and much of 
it has recently been summarized by Hubrecht, who makes much of the 
character of the placentation and derives from it and other features 
some remarkable conclusions. Hubrecht abandons the idea of the 
derivation of the mammalia from the reptilian-insectivorean stem, but 
on the contrary derives them from amphibia-like animals of the Car- 
boniferous. The character of the placentation, moreover, places man, 
the Anthropomorphe and the hedgehog among the most archaic of 
living mammalian types, an idea also borne out by comparative anatomy 
and one which paleontology may some day confirm. 
The most primitive mammals, the Prototheria, are still very sug- 
gestive of their old reptilian ancestry in many ways, especially in the 
method of producing the young inclosed within an eggshell. The posi- 
tion of the Marsupials is surely low in the scale of mammalian life, 
although they show in many respects remarkable specializations. Wilder 
compares them with the Prototheria in that they also bring forth their 
young at a very early state of development, though not protected by an 
eggshell. The period during which they are permanently attached to 
the nipples within the pouch is actually post-embryonic and properly 
larval. Vestiges of placentation have been found in at least one mar- 
supial, a fact which gives color to the belief that in this respect they 
may be degenerate rather than primitive. Broom has shown that the 
modern marsupial shoulder girdle passes through a prototherian stage 
implying a relationship which is strongly supported in other ways. 
The foetal Sirenia and Cetacea, so far as known, show no greater 
development of hind-limbs than do the post-natal individuals. They 
do show a marked neck construction and the head bent at right angles 
with the trunk in a normal quadrupedal posture. ‘The head of the 
foetal manatee is very suggestive of the modern ungulate. Ryder has 
tried to prove the homology of the caudal flukes in the Sirenia and 
Cetacea with the integument of the hind feet, drawing his evidence 
largely from comparison with the seals. In the embryo the flukes 
appear as lateral expansions near the end of the tail, giving it a lance- 
like form when viewed from above. These flaps by transverse expan- 
