152 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 papillae 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 Anthropomorphae 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 

 fcetal 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- 



