GASTRULATION AND EARLIEST DEVELOPMENT 216 



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 it is 

 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. 



Notochord. — 'Whai are we to think of the notochord? 

 Must it be considered as endoderm or as mesoderm? 

 Nothing of the kind is found in Annelids, evidently the 

 notochord is an organ that has been acquired only at the 

 transition of the Annelid into the Chordate. Formerly (1913, 

 p. 696) I 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 a 

 longitudinal ridge of entoderm in often squeezed in, as it 

 were, and separates them. This is shown e.g. in the 

 transverse section in fig. 45 and still more evident I found 

 it to be in transverse sections of 5co/op/os(DELSMAN, 1916, 

 a, figs. 35, 36, 44). 1 imagine this ridge of entoderm to 

 have split off from the roof of the archenteron and to have 



