282 



EMBRYOLOGY. 



I. The Formation of the Mouth, the Throat- or Gill-Clefts, and 



the Anus. 



At the beginning of development the alimentary tube opens out to 

 the surface of the germ by means of the primitive mouth (primitive 

 groove), which marks the place at which, during the stage of the 

 ^blastula, the inner and middle germ-layers have been invaginated 

 (Chapters V. and VI., figs. 44, 47, 54, 55, 78 u). But this opening 

 is only a transitory structure. 



Located at the future hind end of the embryonic fundament, it 



is at first overgrown by the 

 medullary ridges, and es- 

 tablishes a temporary union 

 between the intestinal and 

 neural tubes, the canalis 

 neurentericus (figs. 68 en, 

 80, 88 ne). Afterwards it 

 becomes entirely closed by 

 the growing together of 

 the edges of the primitive 

 mouth. 



It is affirmed by some that in 

 certain Vertebrates (Petromy- 

 zon, several Amphibia) the 

 primitive mouth persists, and 

 becomes the anus of the adult 

 animal. 



There arise, however, on 

 the permanent alimentary 

 tube, both at its anterior 

 and posterior ends, new 

 openings, part of which are unpaired, part paired ; for the wall of the 

 alimentary tube at several places fuses with the wall of the body, 

 then becomes thinner, and finally breaks through to the outside. 

 The unpaired openings are mouth and anus ; the paired ones are the 

 throat-, gill-, or visceral clefts. The first to be established are the 

 mouth and the gill-clefts, in the regions of head and neck. These 

 are of the greatest importance in the external morphology of the 



* [Huxley has employed metencephalon and myelencephalon instead of 

 epencephalon and metencephalon for the fourth and fifth regions of the brain 

 respectively.] 



ck 



Fig. 151. Median section through the head of an 

 embryo Babbit 6 mm. long, after MIHALKOVICS. 



rh, Membrane between stoniodaeum and fore gut, 

 pharyngeal membrane (Rachenhaut) ; hp, place 

 from which the hypophysis is developed ; h, heart ; 

 led, lumen of fore gut ; ch, chorda ; v, ventricle 

 of the cerebrum ; if, third ventricle, that of the 

 between-brain [thalamencephalon] ; t>*, fourth 

 ventricle, that of the hind-brain and after-brain 

 [epencephalon and metencephalon,* or medulla 

 oblongata] ; ck, central canal of the spinal cord. 



