PRIMITIVE ORGANS OF THE VERTEBRATE. 331 



In the important transverse section through the germ- 

 shield of a Chick (Fig. 99), which represents these primitive 

 organs in their original relative positions, they are seen to 

 be flattened and spread out ; and they are found in this 

 same condition in a corresponding transverse section through 

 the germ-shield of a Mammal. In order rightly to appre- 

 ciate these instructive sections (with which Figs. 3 and 4 

 on Plate IV. should be compared), it must be remembered 

 that the layer-like extension of the flat germ-layers over 

 the surface of the large yelk-sac represents a derived, 

 kenogenetic condition, which has arisen in consequence of 

 the gradual acquisition of a large nutritive yelk. In those 

 low Vertebrates in which there is no such yelk-sac, and in 

 which the original, palingenetic condition is more or less 



j. f t 



Fro. 99. Transverse section through the germ-shield of a Chick (on the 

 second day of incubation, about 100 times enlarged). In the outer germ- 

 layer the axial dorsal furrow has completely closed and forms the medullary 

 tube (mr), which has separated itself from the horn-plate (h). In the 

 middle germ-layer the axial notochord (c/i) has entirely separated itself 

 from the two primitive vertebral cords (uw), in the interior of which a 

 transitory cavity (uwh) afterwards forms. The side-layers have split into 

 the outer skin-fibrous layer (hpl) and the inner intestinal-fibrous layer 

 (df), which are still connected by the middle plates (mp). The fissure 

 (sp) between the two is the rudiment of the body-cavity. In the gap 

 between the primitive vertebral cords and the side-layers on either side are, 

 attached on the outer side, the primitive kidney (ung), on the inside the 

 primitive artery (ad). (After Kolliker. ) 

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