1895.] Br Gaskell, The Origin of Vertebrates. 21 



cranium, until at last in Ammocoetes a large cranial and spinal 

 cavity containing a small brain and spinal cord surrounded by a 

 large mass of peculiar glandular tissue. One step further, enlarge 

 the extent of this glandular tissue and make it functional and we 

 find immediately that in position and relative distribution we are 

 face to face with the old cephalic liver and generative organs of 

 the Limulus and Scorpio, and that the same gradual evolution 

 which has explained the nervous system itself gives the same ex- 

 planation of the peculiar tissue which originally fills up the greater 

 part of the cranium and is gradually squeezed out of existence as 

 the brain grows. 



This characteristic glandular tissue, the remains of the old 

 generative glands and liver of the Limulus, is found nowhere else 

 in the Ammocoetes except in the auditory capsule, where its 

 presence will be explained later. 



The history of the Vertebrate skeletal system. 



Passing from the nervous system let us now consider the 

 skeletal system. 



In the highest Vertebrates a bony skeleton enclosing the 

 nervous system to which is attached a facial skeleton, a thoracic 

 skeleton and bones for the extremities. This bony skeleton can 

 be traced step by step into the bony skeleton of the fishes and 

 thence into the cartilaginous skeleton, where it is then found to 

 form a cartilaginous cranium and vertebras from which spring 

 ventrally a series of cartilaginous bars associated with the branchiae 

 and viscera. Further downwards we can trace the whole system to its 

 simplest beginning in the Ammocoetes, where we find the cranial 

 cartilaginous skeleton consists of the basal trabeculae, in front of 

 which are the nasal cartilages, the parachordals with the auditory 

 capsules and the branchial bars with a cartilaginous continuation 

 of the parachordals along each side of the notochord. We may 

 then conclude that the immediate ancestor of the Vertebrates 

 possessed a simple cartilaginous skeleton consisting of a cartila- 

 ginous bar on each side of the axis which was in connection with 

 an auditory capsule, extra-branchial bars and a nasal cartilage. 

 We can go further than this, for the cartilages in Ammocoetes are 

 of two kinds, hard and soft, the matrix of the first stains yellow 

 with picro-carmine, and of the second red, and we find that the 

 cartilages of the branchial basket-work and of the two disjointed 

 rods along the notochord are of the soft variety, while the auditory 

 capsules and trabeculse are of the hard variety. Further, when 

 transformation takes place the new cartilages are all of the hard 

 variety so that the evidence points to the soft cartilage being older 

 phylogenetically, a conclusion which is in accordance with Shipley's 



