INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 647 



rate, of the sensory cells of the ectoderm must be sensitive to luminous 

 imj)ressious. 



In conclusion, attention is directed to the bearing of the facts 

 detailed on the germ-lajer theory. The authors give a sketch of the 

 change in thought which has had for its effect to give a general mean- 

 ing to the words ectoderm, endoderm, and mesoderm ; but this change 

 has hardly been completely accurate. Let us take as an example the 

 term mesoderm. The students of the embryology of the higher 

 animals apply the word to a layer of embryonic cells ; and they show 

 that these cells become converted into definite tissues and organs. 

 On the other hand, in the Coelenterata, the word is applied to a 

 definite layer of tissue, which is developed between the inner and 

 outer epithelial layers. The matter may be best put thus : In 

 the lowest divisions of the Metazoa there are only two layers, the 

 ectoblast and endoblast ; in the higher there is a third embryonic 

 layer, the mesoblast. These three terms should be confined to the 

 layers of the embryo, and should only be regarded as exhibiting topo- 

 graphical relations. The terms endoderm, &c., should be thus used : 

 By endoderm and ectoderm we mean the outer and inner layers of 

 the developed body, which have been developed from the ectoblast 

 and endoblast of the germ, and have retained their jirimitive position ; 

 the term mesoderm is applied to the sum of all the tissues and organs 

 which are interpolated between the bounding layers, and these may 

 be cither dcrivates of a special mesoblast, or have taken their origin 

 directly from one of the two primary germ-layers. With these defini- 

 tions we can formulate the two following laws : — 



(1) As an animal increases in complexity of organization the size 

 and complexity of the mesoderm increase, while the ectoderm and 

 endoderm become more simple. In the Ccclenterata the ectoderm 

 and endoderm fulfil the most varied functions of the animal, but in 

 the rest these functions are taken on by the mesc derm. 



(2) All the organs which in the higher orders are mesodermal, 

 belong in the lower animals (with the exception of the vascular system, 

 &c. — direct dcrivates of the mesoderm) to the two primitive cell- 

 layers. 



The facts detailed in this paper would, even if unsupported by 

 other similar facts, be suiTicicnt to demonstrate that, when we examine 

 the question of the homology of the layers within the difiercnt 

 divisions of the animal kingdom, we find that the germ-layers 

 undergo different kinds of differentiation. This docs not affect 

 the general homology of the layers ; how docs it bear on the <|uestion 

 whether the twu layers have always the same relations to the tissues 

 derived from them ? After a review uf a number of tlie facts which 

 bear on the question, the autliors come to the conclusion that the 

 germ-layers are neither organ(dogical nor liistological unities. Wo 

 cannot argue from what we know of the develoiunent of an organ 

 in one pliylum as to its history in another.* The steni-form — the 

 gastriea — must not bo regarded as orgauologically and histologically 

 indifferent ; its descendants may have had tlieir tissues and organs 

 * Compare witli this M. FolV views, mitr^ p. (iO"). 



