DEVELOPMENT OF THIRD DAY 



491 



forming a fissure in the stalk 

 along which, and in which 

 the optic nerves and blood- 

 vessels come to lie. This is 

 on the ventral surface of the 

 stalk. In the meantime, the 

 optocoele has practically be- 

 come obliterated, a very 

 small portion alone remain- 

 ing between sensory and 

 pigment layers in the optic 

 cup. Even these fuse short- 

 ly, and then the optocoele 

 disappears entirely. 



The eye lens arises in- 

 dependently of the optic 

 vesicles in the ectoderm, 

 close to the vesicle. At 

 forty hours the ectoderm in 

 this region has thickened. 

 The placodes thus formed 

 grow toward and into the 

 optic cups after themselves 

 forming vesicles. The super- 

 ficial ectoderm from which 

 they arise soon closes again 

 at the point where the lenses 

 have arisen, although a very 

 small opening may still be 

 seen for a short time. 



It is well to call partic- 

 ular attention at this point 

 to the similarity of the way 

 in which the lens of the eye 

 and the auditory vesicle de- 

 velop by a thickening of 

 ectoderm, then insinking 

 and finally completely sep- 

 arating from the superficial ectoderm from which it sprang. The lesson 

 to be brought home, is that once these structures have separated from 

 the superficial ectoderm, regardless of their original similarity, each fol- 

 lows a totally different line of development and differentiation so as to 

 become structurally and functionally unlike in the adult condition. This 

 original similarity and adult divergence should be noted throughout em- 

 bryological and comparative studies. 



Fig. 288. 



Diagrams showing brain development in vertebrates. 

 Longitudinal sections. 



I. Before the blastopore closes. 



II. At the time three brain regions can be seen. 



III. At the time five brain regions have formed. 

 (Compare with Fig. 281.) 



A, prosencepnalon ; aa, dividing line between telen- 

 cephalon and diencephalon ; c. cerebellum ; cc, cerebellar 

 commissure; ch, (in I and II) dorsal nerve cord; (in 

 III) habenular commissure; en, neurenteric canal; cp, 

 posterior commissure ; cw, thickening on optic nerve 

 due to the crossing of fibers (this is the chiasma) ; D, 

 diencephalon ; dd, line separating diencephalon and 

 mesencephalon; e, epiphysis ; e', paraphysis ; ect, ecto- 

 derm ; ent, entoderm ; ff, line dividing mesencephalon and 

 metencephalon ; J, infundibulum ; It, lamina terminalis ; 

 M, mesencephalon ; Ml, myelencephalon ; Ms, spinal chord ; 

 Mt, metencephalon ; np, neuropore ; P, prosencephalon ; 

 pn, neuroporic process ; pv, ventral brain-fold ; R, 

 rhombencephalon ; r, thickening of ectoderm which is 

 sometimes said to be the anlage of an unpaired olfactory 

 groove ; ro, optic recess; si, the groove (sulcus intraen- 

 cephalicus) which forms the hindermost boundary of the 

 midbrain ; T, telencephalon ; tp, tuberculum posterius 

 (After von Kupffer.) 



