EMJUtTOLOCT. 



.sidled opening, which is surrounded by /* ridge*. A knowledge of 

 theae is of great importance in Studying the history of il* formation 

 of tibef^ 



Of the five ridges one i* impaired, the /rwila/ or *M*0/rfK4a/ 



broad, roujided projection which bounds the oral pit abor* . 

 It* origin k connected with the development of toe cental nervous 

 m, which reaches op to the anterior end of the cwbiyonic 

 fiii*UnM*nf, wfaflfe it is devdopad into tbe cwcbnJ veticie* (fig. 163 

 gh t cA. mA). yiw^ffyifM^p by ffM^ffg of a |^ffjg^t^>n^| tarlim^ tlW 

 frontal prooaae at tbic *t*ge, tb^refore, ntjcioaai A U/ge ca \ic y be- 

 longing to tbe neoiml tube, and ba tbe form of a vaade, wbieb i* 

 oompoaed of tbree layers, tbe epidenni** a layer of meaencbyway and 

 tbe tbkkened epitbebal vail of tbe neural tube. The primary oral 

 cavity and tbe fundament of tbe brain are dasdy appoaed at tbe 

 beginning of development : tbey are separated by only a tbin dbeat 

 of t issue, witbin whose territory then is fnbmynufiUly formal, amomj 

 other tiling*, the floor oftk* cranium. 



The four remaining ridges are paired Gtructurae which surround 

 tbfr oral ^?F?ii upon its eidffi and below. The^e are produced by 

 growths of tbe embryonic connective tissue, through which large 

 blood-vasseU take their course. They are dkjtinguifihed aooording to 

 their portions a* upper-fa* (maxillary) and /Uwr^w 



Xb6 former are on ^t^bfly ^Af^ in ffffff^*ffalfo contact with 

 tbe frontal prooes^ from which they are aepa/mted by a groove only, 

 tbe naao-optic furrow, which will be diflPiMSfld in a eubseq uent chapter, 

 and which run* obliquely upward and outward to that region of the 

 face in which tbe eye begins its dovnlopmnfit. The maxillary prooass 

 is eaparated from tbe mandibular process by an tndaon which corre- 

 sponds to the place of the future angle of the mouth. The two 

 prommmr of either ade together form the pharyngeal arenas, or the 

 membranous jaw-arches. 



Before the rupture of the pbarynga*! membrane the oral ana* bac 

 till deeper, but only in its upper part, whereas toward the mandibolar arch it 

 shallow. Tbit condition is connected with cw*at*re* which in aU 

 F well a* Selachian* aftect that part of the head which 

 the brain-reticle* and lie* above the alimentary tvbe. For the front 

 and of the head i* bent down toward the ventral /i* of the embryo, and 

 finally make* a right angle with the po*erior half of the bead (fig. 153). 

 Gonaeqnently the place at which the ao-called aferir <*pi*ht **mt*** ha* 

 occurred, and at which tbe posterior and anterior halve* of the head bend into 

 each other, ha* become an elefation, tbejMarMtoJ [orOTtfHfc^]rfMwN**(6che&. 

 telhocber). SB. The latter ancloeat tbe middle brain-reticle <A) the fmlm 



