EARLY ONTOGEXETIC PHEXOMEXA IX MA5IMALS. 223 
[By the way, on p. 4 Hubrechfc Avrites: ‘'There seems to 
be hardly any doubt that both in Ainphioxus and in man — 
the tAvo opposite exti'emes in the phylum of the Chordata — 
the two first cleavage cells, if separated from each other, may 
under favourable conditions each of them develop into a 
perfect, full-grown individual.” Wilson, Morgan, and others 
have shoAvn that this may be true of Amphioxus, but Avhat 
evidence is there that in man the division that gives rise to 
homologous tAvins occurs at this early stage of development? 
Some years ago I found a case of tAvinning in the sheep (’98) 
Avhich I believe is the earliest case known among mamniids, 
and the evidence from that specimen tends to shoAv that the 
division Avhich results in tAvinning may occur at a later period, 
namely, during the formation of blastocyst cavity. I may 
mention that I recent!}" found in the ferret a condition Avhich 
at first sight I took to be a similar case; but investigation 
by sections shows that it is probably not a case of twinning 
though it may be derived from a bi-ovular follicle after the 
manner of the pluri-o\’ular follicles of some Edentates.] 
On Hubrecht’s hypothesis that the trophoblast is derived 
“from a larval layer, an Embryonalhiille ” comparable to 
those of Desor’s laiwa, the Pilidium, or the Sipuncnlid larva 
(p. 17), it is clearly convenient to shoAV that the trophoblast 
originates by delaminatiou as suggested by the figs. 2, 3, G, 
mentioned above, producing a typical Embryonalhulle like 
the hypothetical figure of Hubrecht (’95, fig. li, Taf. Ill), of 
the originating trophoblast. This also is the AA"ay in Avhich 
arise those superficial layers of anarania, Avhich Hubrecht sub- 
sequently — though, as I hope to show, in some cases quite 
erroneously — claims as homologous to the mammalian tropho- 
blast. 
On the other hand such cases of segmenting mammalian 
ova as those Avhich supply evidence of the origin of the outer 
layer by epibole are inconvenient, and this is a point Avhich 
surely should have been considered A"ery carefully, because it 
is opposed to the method of formation of the supposed homo- 
logous layer of the anamnia, and because it suggests an 
