EMBRYO. 193 



towards the median line, and with M. Richerand, to say that the 

 embryo is at first only a groove, the edges of which vegetating 

 from behind forwards, can only unite by a species of suture upon 

 the anterior median line. But attentive and frequently repeated ob- 

 servation compelled me to abandon this hypothesis; in the median 

 line of the face and neck I never could discover any void; I have 

 found it to be as completely closed at the twentieth as at the six- 

 tieth day; nor have I even seen the thoracic organs entirely expos- 

 ed, although the mass from which they grow seems to be covered, 

 as far as the belly is concerned, only with a very delicate tissue; the 

 parieles of the chest nevertheless exhibit their natural appearances 

 as soon as they can be discovered. 



§. II. Of* the Kead, and Org^anis oi* the Senses. 



At the beginning the head resembles a very long club; its subse- 

 quent growth is proportioned to that of the rachis; but the appear- 

 ance of the abdomen and thorax soon occasions it to lose a portion 

 of its apparently enormous size. As neither the face nor chest 

 exist at first, there is in fact no neck at the commencement of 

 embryo life. At five weeks the face is very distinct from the cra- 

 nium, and the head, quite isolated, no longer resembles a mere pyri- 

 form enlargement; and its cranial portion also permits us, most 

 commonly, to discern in the vesicle which it constitutes, the general 

 arrangement of the encephalon. Its facial portion is already quite 

 opaque. 



483, The mouth. The mouth is the first organ of the senses that 

 can be perceived: I have found it in the youngest embryos that have 

 fallen under my notice; consequently, it exists at the twentieth day, 

 and then forms a very large, elliptical or triangular opening; as the 

 upper jaw is very projecting, while the lower one, on the contrary, 

 is very short, it follows that the mouth of a human embryo bears a 

 striking resemblance to that of a young snake. Anatomists have 

 never altered the ideas they had formed as to the manner in which 

 the lower lip is constitu;ed; they have all supposed that it was pri- 

 mitively composed of two lateral portions, which at length united at 

 a middle line, like the two bony pieces which support it; but it is 

 not so with the upper lip. As long as it was thought that the upper 

 jaw consisted only of two pieces, it was supposed that the corres- 

 ponding lip must be also formed of only two pieces. But since an 

 intermaxillary bone has been discovered in the human skeleton, it is 

 generally agreed that the lip is formed of three portions, one middle 

 tubercle and two lateral parts, which by their union give birth to 

 the two columns, or naso-labial ridges. This is the theory upoa 



