DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



151 



change rapidly, so that in embryos of 9-10 mm., the third and fourth arches have 

 sunk inward, thus producing a depression known as the precervical sinus. 

 Soon after this the second arch enlarges, grows over the sinus, and, fusing with 

 the underlying arches, fills up the depression. 



The ventral end of the first arch fuses with the ventral part of the second 

 across the ventral part of the first groove. The dorsal part of the first groove is 

 thus left open and becomes the external auditory meatus. A part of the second 

 arch, together with a part of the first arch bounding the first groove on the 

 cranial side, is transformed into the concha of the ear (Figs. 123, 125, 126). 



The first branchial arch becomes the 

 largest and undergoes profound changes 

 which are extremely important in the de- 

 velopment of the face region. Earlier in 

 this chapter (p. 143) it was stated that the 

 cephalic flexure caused the fore-brain to 

 project ventrally at a right angle to the long 

 axis of the body, and that between the pro- 

 jecting fore-brain and the heart a distinct 

 depression or pit — the oral fossa — was pres- 

 ent. Soon after the appearance of the first 

 arch a strong process — the maxillary process 

 — develops on its cranial side (Fig. 122). 

 The main portion of the arch, which may 

 be now called the mandibular process, 

 rapidly increases in size, extends ventrally 

 and finally meets and fuses with its fellow 

 of the opposite side in the midventral line 

 (Fig. 134). The result of the enlargement 

 of the first arch and its process is that they 

 are interposed between the heart and the 

 fore-brain vesicle, thus bounding the oral 

 fossa laterally (Fig. 122). During this time the heart is gradually moving 

 caudally. Meanwhile a process— the naso-frontal process— grows ventrally 

 from the medial portion of the fore-brain region and comes in contact laterally 

 with the maxillary process. Along the line of contact a furrow is left, which 

 extends obliquely to the region of the optic vesicle and is known as the naso- 

 optic furrow (Fig. 134). 



The various structures which have been mentioned bound the oral fossa 

 which has become a deep quadrilateral pit. Cranially (above) the fossa is 

 bounded by the broad, rounded, unpaired naso-frontal process; caudally (below) 

 it is bounded by the mandibular processes; laterally it is bounded by the maxil- 



Fig. 133. — Human embryo of 4 months.. 

 Natural size. Kollmann. 



