in.] DEVELOPMENT. 23 



As a general rule all the processes (except the hypapo- 

 physes) arise from the part of the vertebra situated above 

 the neuro-central suture, but there are notable exceptions. 



The body of the vertebra is nearly always completed by 

 the addition of a thin disk-like epiphysis at each end, which 

 for a considerable period after it is fully ossified remains 

 adhering by a rough surface to the central or main part 

 of the body, and is easily separated from it by maceration. 

 Its coalescence with the remainder of the body, especially in 

 the thoracic region, is one of the last acts in the completion 

 of the bony skeleton, and does not take place until after all 

 the epiphyses of the limb bones are firmly united. Hence 

 it may be taken as a safe indication that the animal is 

 thoroughly adult. 



It must be noted that the epiphysis covers the whole 

 surface of the end of the body, whether ossified from the 

 centrum or the arch, and is therefore quite independent of 

 the position of the neuro-central suture. 



These terminal epiphyses to the bodies of the vertebrae 

 are peculiar to the Mammalia, but not found universally 

 throughout the class, as they are wanting, or, at all events, 

 very imperfectly developed, in the Monotremata and the 

 Sirenia. In man, the highest apes, and also in some of the 

 Marsupialia, they have less solidity and importance than in 

 other Mammals, being often mere thin osseous rings, repre- 

 senting the circumferential portion only of the ordinary 

 epiphysis. 



The various processes of the vertebrae have been divided 

 into those that are autogenous, or formed from separate 

 ossific centres, and exogenous, or outgrowths from either of 

 the just-mentioned primary vertebral constituents. 



There can be no doubt but that an autogenous process 

 of one vertebra of an animal may be serially represented 



