THE STAGE OF THE FCETUS. 



63 



Anlage producing 

 nasal tip 



Nasal groove 



Naso-optic groove 



Oral surface of 

 maxillary process 



Dorsum of nose 



Lateral nasal pro- 

 cess 



Mesial nasal pro- 

 cess 



Maxillary process 

 of first arch 



Roof of oropharynx 



Portion of head of human embryo of about thirty-four days, showing 

 roof of primitive oral cavity. X 10. {After His.) 



which leads from the nasal pit to the angle of the eye ; this furrow, however, merely 

 indicates the position of the naso-lachrymal duct which develops independently at the 

 bottom of the primary groove. Reference to Figs. 74 and 75 emphasizes the fact 

 that the nasal pits and the primitive oral cavity are for a time in widely open com- 

 munication ; towards the close of the sixth week, however, the maxillary processes 

 of the first arch have approached the mid-line to such an extent that they unite with 

 the lateral margins of the fronto-nasal process as well as fuse with the lateral nasal 

 processes above. Owing to this union of the three processes, the nasal pits become 

 separated from the oral cavity, 



and with the appearance and Fig. 76. 



completion of the palatal sep- 

 tum the isolation of the nasal 

 fossae from the mouth is ac- 

 complished. The lateral nasal 

 processes contribute the nasal 

 alae, while from the conjoined 

 mesial nasal process are devel- 

 oped the nasal septum and the 

 bridge of the nose in addition 

 to the middle portion of the 

 upper lip and the intermaxil- 

 lary segment of the upper jaw, 

 the superior maxillary part of 

 the latter being a derivative of 

 the maxillary process of the first arch. Arrested development and imperfect union 

 between the maxillary processes and the fronto-nasal process result in the congenital 

 defects known as harelip and cleft palate, the degree of the malformation depending 

 upon the extent of the faulty union. 



The Stage of the Foetus. — The fifth week marks the completion of the 

 period of development during which the product of conception has acquired the 

 characteristic features of its embryonal stage ; beginning with the second month and 

 continuing until the close of gestation, the succeeding stage of the foetus is distin- 

 guished by the gradual assumption of the external features 

 which are peculiar to the young human form. In addition to 

 the already mentioned changes affecting the visceral arches 

 and frontal process in the development of the face, the fifth 

 week witnesses the differentiation of the limbs into segments, 

 the distal division of the upper extremity exhibiting indica- 

 tions of the future fingers, which thus anticipate the appear- 

 ance of the toes. The liver is already conspicuous as a 

 marked protuberance occupying the ventral aspect of the 

 trunk immediately below the heart. The head by this time 

 has acquired a relatively large size, the prominent cephalic 

 flexure which marks the position of the midbrain being par- 

 ticularly conspicuous. At the end of the fifth week, or the 

 thirty-fifth day, the foetus measures about fourteen millimetres 

 in its longest dimension. 



The sixth \Areek finds the foetus elongated with greater 

 distinctness of the human form, the large size of the head, 

 on which the cervical flexure is very evident, being highly 

 characteristic when compared with corresponding stages of the 

 lower mammals. The several constituents of the face become more perfectly formed, 

 including the completion of the superior boundary of the oral cavity and its separation 

 from the nasal pits by the septum resulting from the union of the fronto-nasal process 

 with the maxillary processes ; the fusion of the latter with the lateral frontal processes 

 now defines the external boundary of the nostrils of the still, however, broad and 

 flattened nose, which lies im.mediately above the transverse cleft-like oral opening. 

 The visceral arches are no longer visible as individual bars, having undergone com- 

 plete fusion. The differentiation of the digits on both hands and feet has so far 



Fig. 



//• 



Head of human embryo 

 of about seven weeks. X 5. 

 {After Ecker.) 



