THE EOEMATION OF THE LAYERS IN AMPHIOXUS. 331 
mammals, then the change from a viviparous to an oviparous 
method of development, which Hubrecht must postulate, is 
totally unthinkable. How should au animal which had once 
adopted the habit of carrying the young in the womb — the 
safest method of development, and the one which rendered 
the parent completely free from the necessity of visiting any 
fixed place for parturition — revert to the dangerous and 
primitive method of laying eggs ? In every other case in 
which viviparity occurs in the animal kingdom we have 
evidence that it has developed out of oviparity, not vice 
versa. Hubrecht lays great stress on supposed indications 
of a specially differentiated layer of ectoderm over the 
embryonic areas of the embryos of reptiles, like Sphenodon 
and of Echnida, as a proof of their descent from viviparous 
forms. Hill expressly denies the existence of such a layer 
in Ornithorhy nchus (38) and the marsupials (16), but 
even if the facts were as Hubrecht i-epresents them it by no 
means proves his case. The ectoderm in all the higher Verte- 
brata is typically many-layered, and that an outer layer 
should prematurely become differentiated is only au antici- 
pation of adult conditions. Even in Elasmobranchii and 
Cyclostomata, which Hubrecht classes together with Amphi- 
oxus, and separates from all other Yertebrata on account of 
tlieir single-layered embryonic ectoderm, eventually develop 
a many-layered adult ectoderm. Natural selection may have 
seized on this tendency when Rauber’s layer was evolved. 
The way 1 have suggested of looking at the evolution 
of Yertebrata, which has the audacity in these days of inno- 
vation to be commonplace, escapes all these difficulties. All 
grades in the development of viviparity are met with amongst 
living reptiles, and it does not I’equire a very violent exercise 
of the imagination to pass from a condition such as is found 
in Zootoca to that found in Ornithorhy nchus, for 
example. 
Rut Hubrecht claims for his view that it enables him to 
explain the origin of the allantois and the amnion as 
embryonic envelo 2 )es in a satisfactory way. He pictures the 
