28 A. A. W. HUBRECHT. 
general feature in the development of 'Teleostomes, Dipnoi, 
and Amphibia. 
Suppose for a moment that we are justified in looking upon 
the Deckschicht of Amphibia and Teleostomes as being in 
reality homologous to the trophoblast of Mammalia and 
Sauropsids—homologous at least in that sense that what is a 
very active and most important layer during the development 
of the viviparous mammals is only a temporary, evanescent 
arrangement in the Ichthyopsids—then we must at the same 
time ask ourselves: is this homology, perhaps, indicative of 
an error into which we may have fallen when adopting Milne 
Edwards’ distinction of the vertebrates in Anamnia and Am- 
niota ? And should we not reconsider whether and how this 
error can be readjusted ? 
At all events the Elasmobranchs, the Cyclostomes and 
Amphioxus show in their early development no traces of a 
Deckschicht and—as we shall see in a later chapter—no traces 
of other organs which are characteristic for the other verte- 
brates. 
In this chapter I had to point to these facts ; in Chapter ITI, 
p. 81, they will be more fully discussed, as a'so in Chapter 
Wag p. 150. 
CHapter I]. Furraer DerveLopmMent OF THE ‘Two GeERM- 
LAYERS OF ‘HE VERTEBRATES UP TO THE APPEARANCE OF 
THE SOMITES. 
I. Mammatia (Mono- anp Di-peLputa). 
1, Developmental Processes in the Entoderm. 
The participation of the entoderm towards the formation 
of tissue between the two primary layers in Mammals is 
denied by very high authorities as Kélliker, Selenka, Ziegler, 
Keibel, and others, who hold that material for mesoblastic 
