THE FOURTH WEEK. 495 



of the head, are larger and deeper than before, and are bordered 

 by prominent lips, with somewhat irregular outlines. At the 

 inner and ventral corner of each olfactory pit there is a small 

 but deep notch, oj, with a sharply defined border : from this 

 notch the organ of Jacobson is developed. 



The eyes are very much smaller than in a chick embryo at 

 d, corresponding stage of development. The lens is present as 

 a small circular pit, with an open mouth, oc. 



The auditory vesicles, EI, appear from the surface as a pair 

 of rounded swellings, just above the dorsal ends of the hyoid 

 arches, HY. 



The visceral arches have undergone considerable modifica- 

 tion. The maxillary arch, MX, lies immediately behind the 

 eye ; it is larger than before, but is still much smaller than 

 the arches next behind it. The mandibular arch, MN, is the 

 largest of the series, and is partially divided by a constriction, 

 about the middle of its length, into dorsal and ventral portions. 

 The hyoid arch, HY, is nearly as large as the mandibular arch, 

 and is also constricted across its middle. The first branchial 

 arch, BR.I, lies behind the hyoid arch, and is of much smaller size 

 than this. A still smaller second branchial arch is present, but 

 is not visible from the surface, being overlapped and concealed 

 by the first (cf. Fig. 239, p. 552). 



With regard to the visceral clefts, it is probable that none 

 are open at this, or indeed at any period in development ; but 

 the point has not been determined with absolute certainty. 

 Behind the first branchial arch there is, 011 each side of the neck, 

 a deep pit, the sinus pragcervicalis, SU. This is a blind pocket, 

 (cf. Fig. 239), formed by a process of telescoping, through 

 which the hinder pairs of branchial arches are carried forwards, 

 so as to lie between the anterior arches, instead of behind these. 

 The sinus prascervicalis does not open into the pharynx or into 

 any other cavity, and at a later stage it is obliterated by fusion of 

 its anterior and posterior walls with each other (cf. Fig. 210, su). 



The mouth (cf. Fig. 206) is much wider from side to side than 

 in the earlier stages ; it is bounded in front by the fronto-nasal 

 process, at the sides by the maxillary arches, and behind by 

 the mandibular arches. 



In the body of the embryo, thirty-five pairs of somites, or 

 protovertebrge, are clearly visible ; of these, eight are cervical, 



