ORGANIC FORM AND ARCHITECTURE 



683 



externally. This is most clearly seen in the case of hollow organs, 

 such as swim-bladder, lung, and allantois, which are lined with 

 endoderm and externally enveloped with mesoderm. 



There are few organs, in the usual sense of the word, that are 

 altogether derived from one germinal layer, but the brain may be 

 cited as almost entirely ectodermic ; the mid-gut of round worms as 

 wholly endodermic (without the mesodermic investment that is 

 present in most animals); and the heart as wholly mesodermic. 

 The eye is an instance of the intricate combination of ectoderm 

 and mesoderm ; the liver is an instance of the co-operation of endo- 

 derm and mesoderm; the sense-organs of a jellyfish illustrate the 

 rare organogenic association of ectoderm and endoderm. The 

 gonads or reproductive organs usually appear on the mesodermic 



Fig. 97. 



Two Stages in the Development of a Mammal's Brain. After Kupffer. 

 I, the olfactory lobe; 2, the developing eye; 3, the cerebral hemispheres; 

 4, the pineal stalk; 5, the optic lobes; 6, the cerebellum; 7, the medulla 

 oblongata. In the figure to the right, the same parts (la to ya) are shown 

 at a more advanced stage. 



wall of the body-cavity, but in their essential part they arise from 

 the early segregation of germ-cells, and so need not be thought of 

 in connection with any particular germinal layer. 



SUBSTITUTION OF ORGANS.— This idea, partly embryological, 

 partly morphological, has to do with the way in which one organ 

 or structure may lead on, individually and racially, to another 

 which becomes its substitute. It was especially emphasised by 

 Kleinenberg, and may be best explained by an illustration. In the 

 early stages of all Vertebrate embryos, the supporting axis of the 

 skeleton is the notochord — an endodermic rod folded outwards 

 along the dorsal median line of the primitive gut or archenteron. 

 In primitive types like the lancelet and the lamprey it becomes the 

 persistent skeletal axis of the adult animal. It also persists to an 

 appreciable extent in some fishes, such as sturgeon and mudfish. In 

 most fishes, however, and in all higher animals, the notochord is 



