142 DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 



around the temporary anterior neuropore. More anteriorly, but in 

 the roof of the forebrain, and sHghtly dorsal to a line drawn continu- 

 ously from the notochord, one finds an evagination of the roof known 

 as the epiphysis. This is the forerunner of the pineal body or gland. 

 Anterior to the epiphysis the roof of the brain eventually becomes non- 

 nervous, vascular, and folded as the anterior choroid plexus. Later 

 the habenular ganglia and commissure develop between the epiphysis 

 and choroid plexus. The only sense organs to develop by this stage 

 (2.5 mm. length) are associated with the forebrain, a further indica- 

 tion of cephalization within the central nervous system. They are the 

 optic vesicles, paired primordia of the eye, which begin to develop as 

 paired lateral vesicular evaginations of the forebrain even before this 

 portion of the brain is closed over dorsally. The presence of these 

 vesicles was described previously as being dorso-lateral to the sense 

 plate, in a facial view of the embryo. The connections of these vesicles 

 with the forebrain, at the point of the optic recess, become partially 

 constricted off into tubes known as the optic stalks. 



At this stage the paired olfactory sense organs appear only as 

 olfactory placodes or button-like thickenings of the pigmented surface 

 ectoderm, each slightly ventral and mesial to the corresponding optic 

 protuberances. The auditory placodes may be seen in slightly older 

 stages as similar thickenings of the nervous ectoderm dorso-lateral 

 to the level of the rhombencephalon (hindbrain). 



The Enteron or Gut Cavity. 



The anterior limit of the original archenteron expands both ven- 

 trally and laterally beneath the notochord and the infundibulum of 

 the brain. This expanded cavity will give rise to the foregut and all 

 of its derivatives. The midgut, at this stage, is simply the tubular 

 archenteron dorsal to the mass of yolk endoderm. The hindgut is that 

 portion of the archenteron found in the vicinity of the temporary 

 neurenteric canal. 



The foregut has a prominent median antero-ventral evagination 

 of its endoderm known as the oral evagination. This will make con- 

 tact with the head ectoderm just postero-ventral to the level of the 

 hypophysis. There is an opposed stomodeal invagination of head 

 ectoderm which will meet this endodermal evagination, later to 

 break through as the mouth. When these two germ layers make con- 

 tact, prior to the rupture, they constitute the oral plate. 



