EAELY ONTOGENETIC PHENOMENA IN MAMMALS. 261 
(6) The front end of the primitive streak becomes heaped 
up into a mass of tissue, the protochordal wedge, which, fusing 
with the tissues in front, adds, as the primitive streak recedes, 
deuterogenetic tissues to the notochord, nerve plate and other 
tissues affected by the growth in length. 
(7) The boundary between tlie protogenetic part and 
deuterogenetic part of the notochord may be between the 
part of the notochoi’d (archenteric plate) which is described 
by the authors as having a smooth under-surface and the part 
to which pieces of the broken-down “ Mesoderrasackchen ” are 
attached. 
(8) A neurenteric canal is here formed as in some other 
mammals and birds, by the interaction of the two great 
growth centres, but on a much more marked scale which is 
exceedingly reptilian in its characters. 
If we accept the author^s nomenclature and regard the 
Mesodermsackchen as archenteron, we are brought back to 
the old difficulty of being unable to account for the sub- 
germinal cavity. If, once for all, we abandon all idea of a 
true blastopore in any Amniote, and regard the subgerminal 
cavity of birds, reptiles, and blastocyst cavity of mammals as 
homologous and all as archenteron, the result of protogenetic 
activity, and if we regard all subsequent pores which are 
invariably connected with growth in length as metenteric (to 
use the term suggested before) and deuterogenetic in origin, 
the matter seems to me to be greatly simplified, and is in 
complete accord with the results of experimental inquiiy as 
to liow things actually do grow in the living embryo. 
ChAPTEKS III AND IV. 
Placentation. 
With the general trend of view in Chapter IV, which is 
admirable in all respects, few will wish to quarrel. It 
contains a clear account of the character of the trophoblastic 
changes in most of the mammalian orders. It is a matter of 
