498 



THE HUMAN EMBRYO. 



the area, formed by a sagittal fold of its surface, and gradually 

 grows forwards to form the bridge of the nose. For some time 

 the nose is very short and inconspicuous, and the nostrils very 

 far apart ; but towards the end of the second month (cf. Figs. 

 213 and 214), the nose grows forwards more prominently, and 

 the nostrils are brought closer together. The alee nasi, forming 

 the outer borders of the nostrils, are developed from the lateral 

 nasal processes. 



The mouth changes its shape very markedly during the fifth 

 week. At the beginning of the week (Fig. 206, DS) it is a 



BS 



OK 



FP 



HM 



FIG. 206. 



FIG. 207. 



FIG. 206. The under surface of the head of a Human Embryo lettered by 



Professor His, Hn, and estimated as about twenty-nine days old. (From 



His.) x 7|. 

 FIG. 207. The under surface of the head of a Human Embryo lettered by 



Professor His, C.I I, and estimated as about thirty- four days old. (From 



His.) x 5. 



BS, cerebral hemisphere. DS, stornatodaeum. 3PO, processus globularis, or lateral 

 portion of fronto-nasal process. FP, median portion of fronto-uasal process. HM, 

 hyomandibular cleft. MK", mandibular arch. MX, maxillary arch. OC, eye. OK, 

 olfactory pit. 



wide opening, extending transversely across the under surface 

 of the head ; but before the end of the week (Fig. 207) it has 

 become greatly reduced in size, by convergence of the maxillary 

 arches and the processus globulares, and is now a narrow trans- 

 verse slit. Between the maxillary arch and the lateral nasal 

 process of each side is a depression, the lacrymal groove, which 

 at first (Fig. 206) leads into the stomatoda3um, but which on the 

 completion of the narial passage opens into this latter (Fig. 207). 

 In the region of the visceral arches and clefts, important 

 changes occur. The tendency of the anterior arches to grow 

 backwards over the hinder arches, or, as it may be better 



