DEVELOPMENT OF THE SKULL IN THE CROCODILLi. 299 



it gives promise and prophecy of the highest of all skulls, viz. that of the Mammal. 

 It is hardly necessary for me to state that I do not consider this last kind as arising 

 from a type directly overhjing the Sauropsida. The Mammalia form another branch 

 of the Aynniota, which has a separate root, and has, on the whole, a much higher cul- 

 mination. In some very important things the skull of the Anurous Amphibian forms a 

 better leading-step to that of the Mammal than any to be seen in Reptiles and Birds. 



The highest of the many branches of the Sauropsidian stock is the Passerine form ; 

 but there is no crossing over from that to the Mammalia possible ; we must slide down 

 the whole of the vertebrate trunk, to its very root, before we are in a position to find 

 the first shoot that grew Mammal-ward ; this, perchance, was quite as low as the point 

 from which the Sauropsida grew. 



Summarij. 



As to the first stage it is scarcely necessary to point out the extreme similarity of 

 the early embryo of the Crocodilian, not only to that of the other Sauropsida, but also 

 to that of every other vertebrate type. 



The cartilage at this stage is becoming solid, the sense-capsules are seen to be all 

 separately formed, and the basis cranii can be made out, although it is in a very 

 primordial condition. As in the Axolotl, the j;rochordal tracts are merely small horns 

 budding out from the large 2J«>'«chordal plates. But the rapid growth of the hemi- 

 spheres, the outgrowing optic vesicles and olfactory lobes — all developments of the 

 vesicular fore brain — is attended with an equal amount of prepituitary skull-growth, and 

 in the second stage the jjrochordal outgrowths are equal in length to the proper axial 

 tracts, or jj«r«chordal plates. These latter run with the notochord into the hollow of 

 the folded mid brain, but not beyond it ; for that azygous axial rod bends downwards 

 a little, as in the Chelonians and Elasmobranchs, but the mesocephalic flexure aflfects 

 it less than it does the overlying brain-mass ; it is arrested in its forward (and upward) 

 growth. This partial arrest or suppression of the front part of the notochord is corre- 

 lated with a great and, as it were, sudden development forwards and upwards of the 

 investing basal cartilage. 



The part which grows iqmards appears to me to be the true end of the paired 

 elements of the skeletal axis ; and it is this part, viz. the large sloping " postclinoid" 

 wall, which gives rise to the neural laminee of the hinder sphenoidal region, the ali- 

 sphenoids. Behind, the occipital ring is directly formed as an upgrowth, right and 

 left, from the parachordal plates, and the want of continuity of the pre- and post- 

 auditory walls is due to the intrusion of the large auditory capsules, which push the 

 fifth and seventh nerves forwards, and the ninth and tenth nerves backwards. The 

 proper termination of the primary neural and skeletal axes appears to me to be just 

 where the " infundibulum " grows down to meet the oral involution (pituitary rudiment), 

 and where the bulbous end of the hook of the notochord is seen. The hemispheres 

 now rest upon the large adze-shaped front wings of the sphenoid (orbito-sphenoids). 



