160 ernst haeckel. 



4. The Phylogenetic Signification of the Four 

 Secondary Germ-Lamella. 



Whereas the homology of the two primary germ-lamellse 

 with the exoderm and entoderm of the gastrula, and their 

 phylogenetic identity in all the groups of animals (except the . 

 Protozoa), may be accepted with tolerable certainty, on the 

 other hand, the comprehension and interpretation of the so- 

 called mesoderm, or of the middle (third) germ-lamella, and 

 of all the parts which spring from this between the two 

 primary germ-lamellae, is still subject to many doubts. The 

 contradictions which exist, in this respect, between the dif- 

 ferent authors are so great and fundamental that it is 

 altogether impossible, in the present condition of ontogenetic 

 literature, to bring them into agreement. Not only is the 

 origin and further development of the middle germ -lamella 

 quite differently described in the differentgroups of animals, but 

 even in one and the same animal (for instance, in the common 

 hen or in the trout) different observers affirm completely 

 opposite facts with equal certainty. One author makes the 

 mesoderm to proceed from the lower germ-lamella just as posi- 

 tively as a second author makes it proceed, from the upper 

 germ-lamella; a third author thinks that one part of the meso- 

 derm arises from the lower, and another part from the upper 

 germ-lamella ; while a fourth author actually makes a portion 

 of the middle germ-lamella, or perhaps even the whole, wander 

 inwards "from the outside!" from the unorganised nutritive 

 yelk. Even if we wish to make excuses for a large part of these 

 irreconcilable contradictions on account of the difficulty of 

 observation, yet the larger portion is certainly due only to the 



the true homology of the intestine in all these animals, and their common 

 descent from the Gastraea, is of such importance, that I will at least reply 

 ■to the most important of the objections which may be raised against it. 

 These objections concern the apparently very different origin of the gastrula 

 from the morula. In most cases a globular cell-vesicle arises from the 

 morula, the wall of which is formed by a cell-membrane. As this 

 vesicle inverts itself at one point, it forms a cup with two surfaces. If this 

 inversion is complete, so that the invaginated portion (entoderm or gastral 

 layer) attaches itself on the inside to the outer, uninvaginated portion 

 (exoderm or dermal layer), the gastrula is complete. This seems to be 

 the original manner of the formation of the gastrula. On the other hand, 

 in other cases the morula hollows itself out internally, and the central 

 hollow (stomachic cavity), the wall of which consists of two layers, subse- 

 quently breaks through externally (oral opening). This mode of formation 

 of the gastrula seems to be abridged from the first by inheritance. Tl)e 

 result is just the same in both cases, and the apparently significant 

 difference of the genesis is secondary, and to be considered as the result of 

 adaptation, as Ray Lankester (1 . c, p. 330) has very well shown. 



