4 8 



THE ORIGIN OF VERTEBRATES 



tissues in the head-region of Ammoccetes. Fig. 141 represents a 

 section through the head near the pineal eye. Most internally is a, 

 a section of the membranous cranium, then comes b, the ran co- 

 cartilaginous skeleton, then c, the laminated layer, and finally d, the 

 external cuticle. If in Ammoco'tes we possess an epitome of the 

 history of the vertebrate, how would these layers be represented in 



the past ages, supposing they 

 could be fossilized ? 



The most internal layer a, by 

 the formation of cartilage and 

 then bone, represents the great 

 mass of vertebrate fossils ; the 

 next layer J>, by a process of 

 calcification, as previously argued, 

 represents the head-shield of the 

 Osteostracan fishes ; while the 

 cuticular layer d, no longer thin, 

 is the remnant of the Palseo- 

 stracan head-carapace. Between 

 these two layers, b and d, lies the 

 laminated layer c. Intermediate 

 to the Paheostracan and the Osteo- 

 stracan comes the Heterostracan, 

 with its peculiar head-shield — a 

 head-shield whose origin is more 

 easily conceivable as arising from 

 something of the nature of the 

 laminated layer than from any 

 other structure represented in 

 Ammoccetes. 



My present suggestion, then, 

 is this : the transition from the 

 skeletal covering of the Paheostracan to that of the highest verte- 

 brates was brought about by the calcification of successive layers 

 from without inwards, all of which still remain in Ammocoetes and 

 show how the external chitinous covering of the arthropod was 

 gradually replaced by the deep-lying internal bony cranium of the 

 higher vertebrates. 



In Ammoccetes the layer which represents the covering of the 







. i 



-& 



Fig. 141. — Section of Skin and Under- 

 lying Tissues in the Head-Region 

 op Ammoccetes. 



a, cranial wall ; b, muco-cartilage ; c, 

 laminated layer ; d, external cuticular 

 layer. 



