20 



Normentafeln zur Entwicklungsgeschichte der Wirbeltiere. 



Frotojtterus 



Stage 



VII 

 IX 



XI 



XIII 



XIV 



XVI 



XVIII 



XX 



XXI 



XXIII 



External features 



Completely segmen- 

 ted. 



Advanced segmen- 

 tation. 



Segmentation 

 cavity 



Chinks between 

 blastomeres. 



Large segmen- 

 tation cavity 

 with thin 2- 

 layered roof. 



At its maxi- 

 mum. 



Almost obliter- 

 ated reduced 

 to chinks be- 

 tween blasto- 

 meres. 



Invagination 



Faint swellings in 



position of external 



giUs and pronephros. 



Medullary folds fused. 



Pronephric swelling 



more elongated. 



Optic outgrowths 



indicated by slight 



bulging of surface. 



Head region pro- 



jecting more distinctly 



above general surface. 



Branchial eminence 

 becoming segmented 

 Tip of head projecting 



markedly: tail end 

 beginning to project. 

 T-shaped pronephros 

 with duct slightly cur- 

 ved. Duct does not 

 yet reach hind end? 



Long longitu- 

 dinal groove. 



Slightly con- 



cave towards 



dorsal side. 



Patch of macro 



meres still ex- 



posed. 



Short crescentic 

 blastopore. 

 Macromeres 

 completely 

 covered in. 



Blastopore 



short latitudinal 



slit. 



Archenteron 



Shallow slit 

 just begin 

 ning to bend 

 dorsalwards 

 at its inner 

 end. 



Rounded at 



its inner end. 



165" 



(in other 



specimens of 



same stage 



down to 65"). 



135" 



255" 



Dorsal 



wall of 

 archenteron 



Cement 

 Organ 



Metotic 

 mesoderm 

 Segments 



290° 



Mesoderm rudi- 



ment on each 



side marked off 



by its more 



rounded cells 



from the more 



compact enteric 



roof and noto- 



chordal rudi- 



ment. 



Notochord not 



yet split off 

 from endoderm. 



About 

 seven. 



Nervous 

 System 



Rudiment 



present as 



slight ticke- 



ning of deep 



layerofecto- 



derm. 



About 10. 



Anterior 2 



with widely 



open myo- 



coeles. 



About 17. 

 Myocoeles 

 developed 

 anteriorly. 



24. 



Inner wall 



cells flat- 



tened, plat- 



like. 



(Deep layer 

 of ectoderm 

 columnar in 



medullary 

 plate region.) 



Medullary 



folds make 

 their appear- 



ance. Me- 

 dullary keel 



well devel- 

 oped. 



Thick solid 

 rudiment of 



brain and 

 spinal cord. 

 Medullary 

 folds have 

 met except 

 posteriorly 

 where they 

 diverge to 

 Surround 

 blastopore. 



Solid. 



Lumen defi- 

 nite in spinal 



cord and 



narrow split 



forming in 



brain. 



