404 THE RABBIT. 



throughout the whole period of development ; it is attached to 

 the dorsal wall of the body cavity by a very short mesenterial 

 fold, and is easily recognised in an adult rabbit. 



A well-developed post-anal gut, or prolongation of the hinder 

 end of the intestine into the tail, is present on the tenth and 

 eleventh days. By the twelfth day (Fig. 150), the greater part 

 of this has already disappeared ; a small diverticulum of the 

 cloacal cavity, GP, marks its basal portion, and detached frag- 

 ments of it may persist for a time at intervals along the tail. 



This post-anal gut is probably a secondary feature, and due, 

 as in the frog, to the drawing out of the alimentary canal into 

 the tail as this latter lengthens. 



6. The Proctodeum. 



The proctodeum in the rabbit is little more than the actual 

 anal opening ; it develops late, and is usually not formed until 

 about the sixteenth day. 



B. Organs Developed in connection with the Alimentary Canal. 



1. The Teeth. 



Teeth are cutaneous structures, developed from the mucous 

 membrane covering the jaws. They appear in rabbit embryos 

 during the third week, and are at first independent of the bones 

 of the jaws ; indeed, the upper teeth develop before the 

 maxillary bones are formed (cf. Fig. 156). The jawbones, how- 

 ever, soon acquire close relations with the teeth, growing round 

 them, and inclosing them in sockets. 



In the rabbit, as in Mammals generally, there are two sets of 

 teeth, known as milk or deciduous, and adult or permanent, 

 respectively. The deciduous dentition of the rabbit is repre- 



2 3 



sented by the formula : — di. - ; dc. ; dm. - ; the correspond- 

 ing formula for the permanent dentition being, 

 .20 3 3 



L P C> 6' Pm - 2' m ' 3- 

 The milk, or deciduous, teeth in the rabbit are lost very early. 

 The deciduous incisors, corresponding to the large chisel-shaped 

 incisors of the permanent set, are very small, and are shed before 

 the birth of the young rabbit. The second pair of deciduous 



