THE GASTRAEA-THEORY, ETC. 163 



muscular threads of the Hydra (the first beginning of the 

 inesoderm) do not become independent cells, but remain as 

 only thread-like processes of the nervous cells of the outer 

 cell-layer, the " neuro-muscular cells." 



It is not intended to imply by this that the mesoderm 

 is always originally composed of these two layers. As both 

 muscular layers subsequently arose independently of each 

 other, the dermal — the cuticular muscular sheath — as an 

 organ of departure for the skin ; the gastral — the intestinal 

 muscular sheath — as an organ of departure for the intestine ; 

 therefore the case is also phylogenetically conceivable that 

 only one of the two developes itself. This is actually 

 the case in some Hydroida, and probably in the majority of 

 the Acalephse; the intestinal muscular layer is here absent 

 and the entire mesoderm is a product of the exoderm, and, 

 therefore, with all its parts, corresponds only to the cuticular 

 muscular layer. 



As the two muscular layers in the axial portions of the 

 body cohere at first in the Vertebrata, and only separate 

 afterwards, we can explain, by a very old process of growth, the 

 four originally separated secondary germ-lamellse, which are 

 found in the axis of the body in the oldest Acrania, and stand 

 in original connection with the origin of an inner central 

 axial skeleton (the chorda). As the germ-lamellse are already 

 early intimately connected in the " axial cord," from which 

 results many an ontogenetic obscuring and abridgment of the 

 original phylogenetic processes, it indicates also the very 

 early differentiation of the chorda and many other special 

 processes, which occur early in these axial portions of the 

 body. On the other hand, we can satisfactorily explain, not 

 only many of these peculiar processes, but also the contra- 

 dictions of most authors by the view that this central " axial 

 cord" is a secondary process of growth, and that subsequently 

 both primary germ-lamellae (in the five higher groups of ani- 

 mals) take part in the composition of the mesoderm. 



By this view the origin of the body-cavity can be very 

 easily physiologically explained. It can be pictured 

 quite mechanically as soon as it is remembered that the two 

 muscular layers just developed have a simultaneous action 

 independently of each other. A division is then necessarily 

 produced between the two, and fluid collects in the cavity 

 thus formed. This fluid transfuses through the intestinal 

 wall into the primitive body-cavity, and is the first blood, 

 and separate cells of the intestinal fibrous layer, detached 

 during the transudation, which remain in this primitive blood- 

 fluid and multiply, are the first blood-cells. 



