124 ANIMAL BIOLOGY. [Part I. 



growth of the bone in thickness is exogenous and periosteal, but 

 in length is endogenous and endochondral. 

 . Independent ossifications taking place at the growing ends of 

 the bone give rise to the epiphyses, which remain separated 

 from the shaft by a pad of cartilage. Growth then takes place 

 mainly in this pad ; its outer (epiphysial) half increasing by 

 cartilaginous growth as fast as its inner (diaphysial) half is 

 invaded by the growth of bone from the shaft. In the adult 

 the whole pad becomes ossified and growth ceases. The whole 

 femur has become bone, except the articular surfaces which still 

 have a cartilaginous layer. 



The Mesenteron. We have seen (p. 105) how, in the chick, 

 which we will here take as our type, the mesenteric tube is 

 constituted during the folding off of the embryo. By the end 

 of the third day of incubation there are three divisions of the 

 mesenteron (1) an anterior division, comprising the tubular 

 portion produced by the union of the splanchnic layers of the 

 cleaved mesoblast in the neighbourhood of, and somewhat 

 posterior to, the head-fold ; (2) a posterior division, com- 

 prising the tubular portion produced by the union of the 

 splanchnic layers in, and slightly anterior to the tail-fold ; and 

 (3) a mid division, not yet converted into a tube, and resting 

 upon the yolk-sac. The first will constitute the oesophagus, 

 crop, proventriculus, gizzard, and duodenum; the second the 

 large intestine and rectum ; the third or mid division the small 

 intestine. 



In the anterior part of the first division four clefts on each 

 side open outwards to the exterior. They are the visceral clefts, 

 similar to those which are seen in the tadpole, Bounding them 

 are five pairs of visceral arches, the anterior pair nearly meeting 

 in the middle line, the succeeding pairs being successively 

 shorter. The last three clefts soon completely close. But the 

 inner ends of the first pair persist as the Eustachian tubes of 

 the adult. 



Immediately behind the region of the visceral clefts a ventral 



