330 
E. W. MACBEIDE. 
In support of it, it is true, Hubreclit figures from Legros an 
oblique longitudinal section of an abnormal Ampliioxus 
gastrula, which is utterly unlike the appearance presented 
by any normal embryo. Legros admits the section to be 
oblique, a fact of which Hubreclit does not apprise his 
readers. Hubreclit regards Rauber’s layer as a special larval 
envelope and utterly distinct from the true ectoderm. He 
imagines that the aquatic ancestor of Mammalia had a larva 
in which there was such an envelope which was afterwards 
cast off, and cites certain Trochophore larvm as analogous 
instances. When the aquatic ancestor took to a land life the 
free-swimming larva was retained within the womb of the 
mother, and so the peculiar development of Mammals was 
attained. According to this reasoning, then either birds 
and reptiles arose from a different stock from Mammals, or 
else the oviparous method of development which they exhibit 
was secondarily developed out of a previous viviparous con- 
dition. Now on this view several remarks may be made. 
Rauber’s layer is not analogous to the investing layer of the 
Sipun cuius larvfe as Hubreclit imagines, because iu the 
latter case we have to do with a median belt of larval ecto- 
derm which develops into a broad ciliated baud overlapping 
the remaining ectoderm before and behind. The loss of this 
belt in the Trochophore larva leaves a wound which is closed 
by the cicatricial union of the ectoderm produced by the 
head and tail blastema respectively, whereas Rauber’s layer 
is an outer layer of ectoderm according to Hubreclit. Then, 
whichever alternative we take of the ancestry of Mammalia, 
we are beset with difficulties. To maintain that they are the 
offspring of a distinct stock from that which gave rise to 
birds and reptiles is a supposition which may be left to the 
tender mercies of comparative anatomists, who will make short 
work of it. Every recent discovery in palaeontology tells 
against such a supposition ; the mammalian vertebral column 
is constructed on the reptilian plan, whilst the amphibian one 
is built on a different plan, and so on. But if we admit the 
existence of a common ancestral stock of birds, reptiles, and 
