165 
the nature and origin of the mesoderm and the notochord 
of Vertebrates. This question has given rise to an almost 
hopeless divergence of opinions as well as much controversy. 
While some derive the mesoblast from the primary endoderm, 
others suppose it to originate from the primary ectoblast 
which they assume to have invaginated round the blastopore 
border, or partly from the ento-, partly from the ectoderm. 
According to some, the mesoderm in Craniates is formed 
by evagination of paired pockets from the archenteron 
roof, others assume a process of splitting off. Some allot 
the notochord to the mesoderm which accordingly has a 
Single nature in the beginning, others consider the notochord 
as belonging to the endoderm and thus imagine the mesoderm 
as a paired band on both sides of it. Some distinguish a 
gastral from a peristomal mesoblast, others hold that all the 
mesoderm originates from the border of the blastopore, 
being thus peristomal. We shall not enumerate all the 
different views; those mentioned above will show sufficiently 
that the facts alone noted in Vertebrates are again insuf- 
ficient to give us the solution of this problem. A comparison 
with Evertebrates will be the deciding factor. 
The segmentation of the 
mesoderm in Vertebrates is 
one of the main points of 
agreement with the Annelids. 
he way: in which the 
mesoderm is formed, how- 
ever, is very different in 
both. In Annelids we see 
the paired mesoderm bands 
Originate from two large 
cells situated at the border 
of the blastopore and no 
doubt belonging to the 
primary endoderm. They 
produce by teloblastic proli- Fig. 45. Transverse section of an 
feration the two mesoderm earthworm embryo, from BERGH, 
bands which grow inwards 1895, p. 154, after KOWALEWSKY, 
1871. 
pari passu with the inva- 
gination of the entoderm 
along which they are situated. Soon afterwards they become 
segmented from in front backwards. When once this stage has 
been reached the resemblance to what we find in Vertebrates 
