lO 



Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 



Stage 



19 



19a 



20 



21 



Series 



100 Trans. 



101 Trans. 



102 Trans. 



103 Sag. 



105 Trans. 



Length 



6 mm 



Age 



6'/j mm 



7 mm 



8 mm 



106 Trans. 



107 Sag. 

 HO Front 



20 daj's 

 10 hrs. 



21 days 

 2 hrs. 



22 days 

 18 hrs. 



9 mm 



Body Form 



23 days 

 10 hrs. 



Outline of body 

 conformstocur- 

 vature of egg; 

 extends over 

 160" of circum- 

 ference of egg. 



Extemally 6 

 pairs of Seg- 

 ments, those 

 in anterior 

 trunk region 

 roughly tri- 

 angulär in 

 cross sec- 

 tions. Myo 

 coele prä- 

 sent. Muscle 

 plate and 

 cutis plate 

 well defined. 

 Clear zone 

 at ends of 

 muscle plate. 



Body Covers 

 180" of circum- 

 ference of egg. 



Head end of 



embryo slightly 



elevated above 



surface of yolk 



Distinct en- 



largement of 



anterior end 



shows optic 



vesicles. Tail 



end enlarged to 



form slight 



knob. 



Embryo curved 

 laterally. Ante- 

 rior half of head 

 free from yolk. 

 Optic vesicles 

 well defined. 



Somites 



Notochord 



Nervous System 



Extemallyio 

 to 1 2 pairs of 

 myotomes. 

 In anterior 

 trunk region 

 muscle and 

 cutis plate 

 better de- 

 fined, other- 

 wise as in 

 6 mm stage. 



Notochord anteriorly 

 a solid rod of cell 

 Extends slightly in 

 front of ear. Posteri 

 orly not free from 

 ectoderm orentoderm. 



Extemally 14 

 to 16 pairs 

 myotomes. 



Head end ele- 

 vated. Optic 

 vesicles larger 

 Mandibular, 

 hyoid, first 

 branchial and 

 common anläge 

 of second and 

 third branchial 

 arches well de- 

 fined. Caudal 

 enlargement 

 morc pronoun 

 ced. Embryo in 



contact with 



yolk over 140". 



Curved late 



rally. 



Externally20 

 to 22 pairs of 



myotomes. 



In anterior 

 trunk region, 

 muscle plate 

 nearer noto- 

 chord. Clear 

 zones at ends 

 of plate 



wider, fib- 

 rillacpresent 

 in myobl.-ists. 



Well defined through 

 out anterior and 

 middle portion of 

 body; extends slightly 

 in front of level of ear. 

 Anterior portion lies 

 in contact with medul- 

 lary tube. Long tract 

 of mesenchyma be 

 tween anterior end of 

 chorda and hypophy- 

 sis; fused with ento- 

 derm and ectoderm in 

 posterior portion. An- 

 lage of sheath. Indi- 

 cations of hypochorda 



Extends from a point 

 slightly in front of ear 

 to tip of tail. Vacuo- 

 lated anteriorly, not 



posteriorly. Sepa 

 rated from neural tube, 

 fore gut and hypo- 



physis by mesen- 

 chyma. Posteriorly 

 notochord, neural tube 

 and entoderm fused 

 into common mass. 

 Unsegmented. Hypo- 

 chorda well defined. 



Eye 



Ear 



Neural tube closed throughout entire 

 length ; enlarged at anterior end 

 Fore brain, mid brain and hind brain 

 indicated. Anlage of telencephalon 

 Fore brain nearly at right angles with 

 mid brain; prominent, oval ni trans- 

 versa section; walls thin, cavit}' large 

 Mid brain long; oval in transverse 

 section ; canal narrow transversely 

 Optic stalk .short, hollow. Chiasma 

 well defined. Torus transversus small 

 Infundibulum moderate size. No 

 plexus. No commissures. Anlage of 

 trigeminal ganglion, also acusticus, 

 facialis and vagus. Spinal cord long 

 oval in section. Anlage of spinal 

 ganglia. Caudally, notochord, dorsal 

 wall of gut and neural tube fuse into 

 common mass of cells. 



Cranial flexure well marked. Divi- 

 sions of brain better defined. Me- 

 dulla longer than fore brain and mid 

 brain together. In roof of fore brain, 

 paraphysial arch, postvelar arch and 

 epiphysial arch formed. Beginning of 

 velum transversum. Trigeminal nerve 

 forming branches. Roof of meduUa 

 a siegle laver of flattened cells ; nerve 

 fibers forming in Gasserian ganglion. 

 Spinal cord oval in section. Canal 

 Wide, constricted at sides. Roof plate 

 thinner than floor plate. Cells still 

 contain large yolk granules. Spinal 

 ganglia have grown down nearly to 

 middle of side of cord. 



Flexure showing in mid brain. Hemi 

 spheres well developed ; walls thinner ; 

 ventricle very large. Beginning of 

 epiphysis. Velum transversum deeper. 

 Infundibulum forming. Optic chiasma 

 well defined, also torus. Roof of mid 

 brain thin, ventricle widest in dorsal 

 portion. Auditory nerve well devel- 

 oped. Trigeminal ganglion has two 

 branches, one running over eye, 

 other anterior to first gill. Peri- 

 pheral band of nerve fibers on ven- 

 tral and lateral surface of medulla. 

 Spinal cord elongated oval in trans- 

 verse section. Canal wide. Roof 

 plate thin, well defined. Ganglia 

 larger. Undifferentiated mass of cells 

 at caudal end of spinal cord. 



Primär}' optic 

 vesicles better 

 defined; grown 

 slightly back- 

 ward ; outer 

 wall flattened 

 and thickened 

 forming anläge 

 of retina ; open 



widely into 

 third ventricle. 

 Anlage of lens 

 as slight thicke- 

 ning of ecto- 

 demi. No meso- 

 derm between 

 lens and vesicle. 



Vesicle larger. 



Beginning of 

 optic cup. Inner 



layer thinner; 

 outer or retinal 



la3'er slightly 



concave. Optic 



stalk somewhat 



constricted. 



Otic vesicle 

 just closed, 



spherical, 

 lies between 

 ectoderm and 



hind brain. 



Cells elon- 

 gated in ven- 

 tral parttheir 



inner ends 



free from 



yolk. Acus- 



tico-facialis 

 ganglion lies 

 ventro-ceph- 

 aladof vesicle 

 and is nearly 

 as large as 

 vesicle. 



Vesicle com- 

 pletely 

 closed, 

 slightly 

 pear-shaped, 

 smaller end 

 dorsal. In 

 contact with 

 ectoderm. 



Optic cup shal 

 low. Optic stalk 

 better defined 

 contains small 

 tubulär lunien 

 which connects 

 third ventricle 

 of brain and 

 cavity of optic 

 vesicle. Lens 

 consistsof sphe- 

 rical mass of 

 cells arranged 

 radially ; still in 

 connection with 

 ectoderm and 

 containing 

 cavity. 



Vesicle 

 pear-shaped, 

 smaller end 

 dorsal, de- 

 tached from 

 ectoderm. 

 Anlage of 

 ductus endo- 

 lymphaticus 

 as Short dor- 

 sal evagin- 

 ation. Thin 



layer of 

 mesenchyma 

 between ve- 

 sicle and 

 ectoderm, 

 also between 

 vesicle and 

 hind brain. 



