246 
laCHAED ASSHETOX. 
mesoblast, including Hubrecbt’s ventral inesoblast, indicates 
the extension of deuterogenetic mesoblast. This is approxi- 
mately shown by the dotted line of the figures in my paper on 
the primitive streak of the rabbit, 1894, though the dotted line 
should have been considerably nearer the “ embryo ” 
(vide Assheton, 1898, p. 246, on sheep). 
Hubrecht, on pages 35-37, disputes the view that there is 
any forward extension of material from the primitive streak 
area to form a Kopffortsatz, and holds that the protochordal 
wedge becomes lengthened, “ not, however, by its sending out 
any ^Fortsatz,^ but by its being, so to say, ‘spun out ^ in 
consequence of the backward growth of the tissue that is 
going to be the notochord,” with which description I am in 
sympathy, for it is the view put forward by me and illus- 
trated by the diagrams on Plate 22 of my 1894 paper on 
the primitive streak. 
At the same time I think this view must be slightly modi- 
fied in accordance with the results obtained by actual experi- 
ment on tlie growth of tlie embryo in the chick (Assheton, 
1896), Avhich show that the “primitive streak” in the bird 
actually becomes converted into the embryo. It may seem a 
rather subtle difference, but really the elongated part of a 
primitive streak such as one sees in bird or rabbit is the 
stretched-out anterior part of the product of the deutero- 
genetic centre of activity rather than the anterior part of the 
deuterogenetic proliferative area itself, this product becoming 
subsequently differentiated directly into mesoblastic somites 
(which are always deuterogenetic), the deuterogenetic part of 
the neural plate, and of the notochord, etc. 
Hubrecht then goes on to describe another growth centre, 
namely that which gives rise to the ventral mesoblast, which he 
says has preceded the formation of the protochordal wedge, 
and refers to four of his figures, 47-50, which, however, do 
not to my mind indicate any marked distinction either iu 
space or time. Surely the so-called third centre of growth is 
nothing more than the equivalent of the ventral lip of the 
blastopore of Amphioxus and other Anamnia, and is the 
