EMBEYOLOGY OF THE SEA BASS. 
229 
In the Ainniota there is such divergence of opinion as to the precise way in which 
the mesoderm originates that Hertwig is able to defend his position with some success 
from this quarter (20). But in the case of the Teleosts there is such remarkable agree- 
ment in the descriptions of the way in which the mesoderm is formed that Hertwig’s 
theory must, I think, be regarded as in opposition to the facts. The conclusion is 
forced upon us that in certain vertebrates evagiuation has been replaced by delami- 
uatiou in the formation of the mesoderm. 
Accepting this conclusion, we naturally look about for a type which suggests a 
transition from the one method to the other. In looking over the figures with which 
Hertwig illustrates the development of the mesoderm in the frog (20, 1882), the sug- 
gestion that there is here a partial delamination strikes one strongly, and is worth a 
word or two. In Triton, according to Hertwig, the invaginate entoderm or chorda- 
eutoblast is wholly used in forming the chorda. The wall of the gut is formed exclu- 
sively by the darm-entoblast. In the frog, on the contrary, the invagiuated layer is 
a wider as well as thicker one, and not only forms the chorda but also a part of tlie 
roof of the gut (Fig. 5). In the figure, a transverse vertical section, the invagiuated 
7ZC 
Fig. 5.— (after Hertwig, 20). Traiisver.se vertical section through frog emhryo to show partial delamination 
of raesohlast — ec., ectoderm; nc., notochord; mes., mesohlast; c/i. c., chorda-entohlast ; d. darm-euto- 
hlast; a. to 0., delamination fissure ; x. to x., junction of chorda-entohlast and darm-entoblast. 
layer is supposed to extend from x to x, at which points the chorda-entoblast, cli. e., 
and the darm-entoblast, d. e,, pass into the mesoblast, mes. Now the figure, as far as 
it is possible to interpret it without having studied the previous stages, indicates that 
the invagiuated layer in an earlier stage formed a continuous mass, stretching from x 
to X, and of about the thickness of the notochord. This mass appears to have broken 
up into notochord, chorda-entohlast, and the proximal parts of the mesoderm plates 
(from a to h). If this interpretation of the figure is a true one, and I am unable to 
understand the figure on any other supposition, then the mesoblast has delaminated 
from the notochord to x. The further growth of the mesoderm, peripherally from x, 
doubtless takes place as Hertwig describes. 
It is very clear that in the frog the development of the derivatives of the primi- 
tive hypoblast does not take place in the simple ancestral manner shown in Amphioxus 
and Triton. The invaginate layer is made a thick one, so that the chorda is not forced 
