EARLY ONTOGEXETIC PHENOMENA IN J1AMMAL8. 235 
exchange the radial for a bilateral symmetry and to separate 
the coelom from the enteron must at one time have character- 
ised certain coelenterate ancestral forms, as has already been 
advocated b}^ Sedgwick (’84) and by myself (’05) on earlier 
occasions. It is not straining the imagination to assume that 
in this line of descent closely related forms may have deve- 
loped, some with, others without, a larval envelope, tem- 
porarily ensheathing the cellular elements that will build up 
the embryo itself, and thus foreshadowing the separation 
among their later vertebrate descendants of such with and 
such others without a trophoblast.” lie then shows how this 
sporadic appearance of larval envelopes occurs in Nemerteans 
and Annelids. AVhy, therefore, not also in Yertebrata ? 
On the assumption of a terrestrial life an animal ‘Avould 
doubtlessly score certain advantages if at the same time it 
became viviparous. . . . And towards the efficiencyof this 
viviparous condition the larval envelope could immediately 
contribute by the mere change of its protective or locomotor 
significance into an adhesive one” (p. 18), and thus we are 
led on to the conception of the origin of the mammalian 
placenta. 
' Hubrecht quotes Mehnert as showing the existence of an 
outer or trophoblastic layer in Sauropsida, e.g. tortoise, 
lizard, and snakes and many birds. But Hubrecht himself 
finds that Mehnert is proving too much, and rejects those 
cases which are inconvenient, but retains one case, that of 
Clemmys, described by Mitsokiiri, and also Sphenodon and 
Cliaimeleo, on the authority of Schauinsland, as being truly 
trophoblastic, and dismisses the rejected ones as cases 
“ distantly comparable to a mammalian epitrichial layer.” 
Of course on the theory I advocate, the trophoblast is of 
Eutherian mammalian origin only, and is not homologous to 
any form of envelope outside the group of Eutherian mammals. 
Turning to the Icthyopsida Hubrecht finds the Embryonal- 
hiille present as the Deckschicht in Amphibia and Ganoids 
and Dipnoi, and “ more unquestionably ” in the Teleosts. 
As regards tlie Teleosts I have myself (’08), in my paper on 
