166 
becomes evident. The similarity of fig. 45 representing a 
transverse section through an embryo of an earthworm to a 
similar section through a young Amphioxus embryo cannot be 
denied. In Vertebrates, however, the mesoderm is not pro- 
duced by teloblasts but split off longitudinally from the 
primary endoderm in a way on which opinions still differ 
but at any rate quite unlike that in which the mesoblast 
of Annelids is produced. Yet, however different this process 
in the two cases may appear, the application of my theory 
necessarily leads us to the conclusion that there must be 
a fundamental agreement between them. In both cases itis 
evident that the mesoderm is of entoblastic origin, that it is 
formed at the border of the blastopore, that the growth of 
the mesoderm bands is terminal and that the segmentation 
proceeds from in front backwards. The main difference is, 
that in Vertebrates the mesoderm bands for some time 
continue to form part of the endodermic archenteron-wall 
and that consequently when they separate from it they have 
already a certain length and the segmentation sets in at the 
same time. In Annelids the mesoderm bands are separate 
from the endoderm from the beginning and grow inwards 
at the same rate as the latter. 5 
Notochord. — What are we to think of the notochord? 
Must it be considered as endoderm or as mesoderm? 
Nothing of the kind is found in Annelids, evidentiy the 
notochord is an organ that has been acquired only at the 
transition of the Annelid into the Chordate. Formerly (1913, 
p. 696) | felt inclined to share with GOETTE (1890, 
p. 24) in considering it as mesoderm which in other 
cases also produces a similar tissue of vesicular cells. 
Further consideration and reflection, however, has made 
me alter my opinion and now the following concep- 
tion seems to me the most probable. The comparison 
with Annelids leads us to the conclusion that the mesoderm 
in Vertebrates also consists of two bands and thus has 
a paired nature. These two bands in Annelids are separated 
a little distance from each other and between the two à 
longitudinal ridge of entoderm in often squeezed in, as it 
were, and separates them. This is shown eg. in the 
transverse section in fig. 45 and still more evident lfound 
it to be in transverse sections of Scoloplos (DELSMAN, 1916, 
a, figs. 35, £6, 44). 1 imagine this ridge of entoderm to 
have split off from the roof of the archenteron and to have 
