EAELY ONTOGENETIC PHENOMENA IN MAMMALS. 247 
ventral portion of the naturally circular deuterogenetic pro- 
liferation area. 
This area is essentially circular in itself, but it assumes a 
different shape by reason of varying- conditions and is a 
circular ring in Anamnia, which have blastopores like Rana, 
of an elongated ring- or bottle-shape (v. Sedgwick, ‘ Quart. 
Journ. Micr. Sci.,' vol. 33, p. 564) in Elasmobranchs, and 
streak-like in many mammals, e.g. rabbit, Carnivora- 
Tarsius and birds, truncate and almost disc-like in other 
mammals, e.g. Mus, Cavia, and reptiles. But in all it must 
have anterior, lateral and ventral margins from which pro- 
liferation of cell material takes place. I cannot conceive on 
what grounds Hubrecht can separate the anterior part (his 
protochordal wedge) and lateral wings from the ventral part. 
The figures he refers to do not support it, and besides, it is 
well known that the mesoblast sheet formed by deutero- 
genesis is a continuous sheet passing posteriorly completely 
round the primitive streak, broken only in front by the 
separation of the notochord. 
The so-called extra-embryonic coelom which develops so 
early in Tarsius and man is probably protogenetic coelom, as 
it is well ventral to the posterior sheet of mesoblast (Hubrecht’s 
ventral mesoblast), figs. 49, 50; and indeed, in the former 
figure, 49, it appears to be distinctly marked off from it. More- 
over, the extra-embryonic coelom develops long before any 
trace of the primitive streak, whether protochordal wedge or 
ventral mesoblast is present. 
Hubrecht seems to have abandoned the position he took up 
in 1890 when writing on the embryology of Sorex. At that 
time he was sure that the middle region of the protochordal 
plate gave rise to the notochord : 
“A most remarkable fact, to which I must now call atten- 
tion, is this, that it is not in the posterior region of the epi- 
blastic shield that the formation of the middle layer and its 
earliest representatives — notochord and lateral mesoblast 
plates — is first inaugurated. It is in the hypoblast that the 
first differentiation occurs, which ultimately leads to the 
