402 . THE BABBIT . 



the hyomandibular groove, hm, is by far the most conspicuous. 

 The further development of the hyomandibular groove, and the 

 mode in which it gives rise to the external auditory meatus, have 

 already been described in the section dealing with the ear (p. 397). 

 The tongue is developed as a swelling in the floor of the 

 buccal cavity ; it commences to form on the twelfth or thirteenth 



Fig. 161. — A Rabbit Embryo at the end of the twelfth day, seen from the 

 right side. The yolk-stalk and allantoic stalk are cut short, close to the 

 body of the embryo, x 9. 



BL, cerebellum. BIVE, mid-brain. SH.1, first branchial arcli. EI, auditory 

 vesicle. HM, hyomandibular groove. H Y, hyoid arch. LA, fore limb. LP, liind 

 limb. MW, mandibular arch. MS, mesoblastic somite or protovertebra. MX, 

 maxillary arch. OC, eye. OP, olfactory pit. TA, allantoic stalk, cut short. TL, 

 tail. VF, fourth ventricle of brain. YS, yolk-stalk, cut short. 



day, and by the eighteenth day (Fig. 151, tn) has attained the 

 form characteristic of the adult. 



The boundary line between stomatodaeum and mesenteron is 

 impossible to fix absolutely, in the later stages of development ; 

 but its position may be approximately determined, if it be re- 

 membered that the stalk of the pituitary body (Fig. 151, pt) 

 marks the posterior boundary of the stomatodseum in the mid- 



