162 ERNST BAECREL. 



appear to show that the same method of development occurs 

 originally and primarily in them, and that the union of the 

 two muscular layers in the simple middle germ-lamella is a 

 secondary occurrence, while the consequent division of the 

 latter into the two first is a tertiary process (vide PI. Vll, 

 iig. 11 — 16, with explanation). In connection with this sub- 

 ject the closest examination of the processes in the axial parts 

 of the vertebrate germ-lamella seems to be specially important. 

 Here all the cell-layers appear to be very early already more 

 or less intimately united to the undifferentiated cell mass, 

 which His designates by the name of the axial cord, and 

 which he considered with the germ-lamella to form part of 

 the original coverings of the embryo. This last view is cer- 

 tainly quite talse. For the division of the two primary germ- 

 lamellse is above all things originally complete, as we learn by 

 comparing the gastrula in the different groups of animals ; 

 and their union in the axial cords of the Vertebrata is to be 

 regarded as a secondary coalescence. But it appears to be 

 a very important observation that this axial cord is composed of 

 cells of the lower and upper germ-lamella, and that it furnishes 

 cells for the lower as well as for the upper lamella of the 

 mesoderm. The fact is also very suggestive, that a hori- 

 zontal division of the side layers, which extend nearly to 

 the axis, takes place also very early indeed, in many Verte- 

 brata just after the separation of the chorda from the side- 

 layers, and even before the differentiation of the layers of 

 the rudimentary vertebrae. In any case this separation of 

 the mesoderm into two middle layers vanishes again during 

 the separation of the layer of the rudimentary vertebrae ; but 

 it is perhaps to be regarded as a precursor of the later per- 

 manent separation of the side layer. As decidedly of im- 

 portance for this question, I might quote the observation of 

 Kowalevsky, according to which only the dermal muscular 

 layer in Amphioxus undoubtedly arises from the outer, while 

 the intestinal muscular layer arises from the inner germ- 

 lamella. Both muscular layers are here completely separated 

 originally (1. c, p. 6, t. ii, fig. 20). Comp. PI. VII, fig. 13. 



If we examine this difficult problem by the light of the 

 * Theory of Descent,^ it appears to be most probable that the 

 cells of the intestinal fibrous layer or of the intestinal mus- 

 cular layer are developed from the cells of the gastral or 

 vegetative lamella in a similar manner to that in which the 

 cells of the dermal fibrous layer or of the dermal muscular layer 

 are originally formed from the cells of the dermal or animal 

 lamella. For the last process the important discovery of 

 Kleineuberg is highly significant, according to which the 



