214 



THE ANCESTRY OF VERTEBRATES 



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. 

 The way in which the 

 mesoderm is formed, how- 

 ever, is very different in 

 the two. 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- 

 feration the two mesoderm 

 bands which grow inwards 

 pari passu with the inva- 

 gination of the entoderm 



aJong 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 



Fig. 45. Transverse section of an 

 earthworm embryo, from Bergh, 

 1895, p. 154, after KOWALEWSKY, 

 1871. 



