THE GASTRAEA-THEORY, KTC. 161 



careless or unmethodical methods of the observers. It is just 

 in the ontogenesis of the mesoderm that it strikingly appears 

 how necessary for ontogenetic investigations is it to constantly 

 look to comparative anatomy and phylogenesis. 



In order to overcome the difficulties which the origination 

 of the middle or motor germ-lamella really presents, it may be 

 advisable, first of all, to separate from the beginning the two 

 really different substances of which it subsequently appears 

 to be composed, namely, first, the outer lamella : Baer's mus- 

 cular layer, Remak's cuticular layer ; (better fibrous cuticular 

 layer) or the muscular cuticular layer (parietal layer of the 

 mesoderm) ; and, secondly, the inner lamella : Baer's vas- 

 cular layer, Remak's intestinal fibrous layer or muscular 

 intestinal fold (visceral layer of the mesoderm). There are 

 very important reasons for supposing that these two lamellae 

 are phylogenetically originally distinct, although they ap- 

 pear ontogenetically in many animals as secondary diffe- 

 rentiations of an apparently simple middle lamella. This view 

 was already maintained by Baer, who makes the two primary 

 germ-lamellae divide each into two lamellae. From the division 

 of the outer or animal germ-lamella arises the cuticular layer, 

 and the muscular layer ; from the division of the inner or vege- 

 tative germ-lamella arises the vascular layer, and the mucous 

 layer. But this view was subsequently almost entirely aban- 

 doned, and it was believed that at first, a third central lamella 

 arises from one only of the two primary cell lamellae, and that 

 the " muscular layer" and the " vascular layer" are products 

 of the division of the latter. 



In any case the first appearance of the mesoderm seems in 

 the Vertebrata to be indistinguishable, so that its entire cellular 

 mass must be derived form one of the two primary germ-lamellae. 

 Only the fact that some of the more reliable observers derive 

 the middle lamella from the upper (animal) cell lamella, while 

 others derive it, in one and the same vertebrate animal from 

 the lower (vegetative) cell lamella, with equal positiveness, 

 gives rise to the suspicion that both the primary germ-lamellae 

 subdivide for the construction of the middle germ-lamella. 

 This suspicion becomes almost a certainty by a comparison 

 with the development of the mesoderm in the different Inver- 

 tebrata, where, in many cases, only the cuticular muscular 

 layer is developed from the upper germ-lamella, whereas the 

 intestinal muscular layer is developed from the lower germ- 

 lamella. Among many observations relating to this, those of 

 Kowalevsky on Euaxes are especially significant. (Petersb. 

 Mem., 1871, vol. xvi. No. 12, pp. 16, t. III). There are also 

 numerous very recent observations on Vertebrata, which 



