Normal Plates of thc Development of Necturus maculosus. 



II 



Nose 



Hypoph3'sis 



Mouth 



Digestive System, Liver, 

 Pancreas, Spleen 



Gills, Thyroid, 



Thymus, Trachea, 



Lung 



Urino- 

 genital 

 System 



Anlasje of 



nasal organ as 



thickening of 



internal layer 



of ectoderm. 



Anlage of hypophysis 



as a long wedge- 

 shaped mass of cells 

 the inner end of which 

 is 2 or 3 layers thick, 



outer end a single 

 layer continuous with 

 deeper layer of ecto- 

 derm. No Capsula. 

 Lies some distance 

 from anterior end of 

 notochord. Wall clo- 

 sely applied to single- 

 layered wall of infun- 

 dibulura. Yolk gra- 

 nules less numerous 

 than in underlying 

 entoderm. 



Position of 



mouth indi- 



cated by 



thickening 



of ectoderm 



Heart and Blood 

 Vessels 



Skin 



Gut extends from level of poste- 

 rior portion of eyes to a point 

 slightly beyond posterior end of 

 embryo. Distended anteriorly to 

 form branchial Chamber: short 

 postanal gut. 



Thickening of 

 internal layer 



of ectoderm 

 more pro- 



nounced and 

 more definitely 

 circumscribed 



Slight con- 

 cavity in sur- 

 face of ecto- 

 derm. Thick- 

 ness of wall ot 



nasal organ 

 equal to that 

 of wall ot fore- 

 brain. Narrow 

 layer of mesen- 

 chymabetween 



nasal organ 

 and forebrain. 

 External laj'er 



of ectoderm 

 can be traced 

 only to margin 

 of nasal organ 



Anlage of thyroid 



as median out 

 growth in anterior 

 portion of floor of 

 iDranchial cavity, in 

 close relation to 



hyomandibular 



arch. No external 



indications of gill 



arches. 



Stage 



19 



Pronephric 



ducts present 



as two short 



straight 



tubes. 



Anlage of liver. 



Single layer ot 



long flat cells 



containing 



coarse yolk 



granules. 



Anlage of heart in Thickened 

 form of a rod of greatly over 

 cells lying below head. Over 

 the Pharynx and body long flat 

 between the ven- cells intersper 

 tral borders of the sed with many 

 pericardialcavities. large oval cells. 



Mass of cells close 

 under infundibulum 

 4—5 cell layers thick. 

 Slight indications of 

 cavity. Has lost con- 

 nection with ectoderm. 



Surface in 

 vagination a 

 broad trans- 

 verse groove. 



Thyroid evagin- 

 ation deeper. Out 



growth longer. 



Mandibular hyoid 



and common an- 



läge of third, 



fourth and fifth 



arches visible 



externally. 



19a 



20 



Gut extends from anläge of mouth 

 to junction of tail end of embryo 

 with yolk; here it runs ventrad 

 and extends somewhat further over 

 the yolk than in the preceding 

 stage. Extends laterally far bey- 

 ond bounds of embryo at level 

 of anterior portion of yolk. Diver- 

 ticulum of gut just behind liver 

 extends ventrally half way to ven- 

 tral surface. Pharyngeal Chamber 

 large. Midgut narrow dorso-ven- 

 trally, wide' laterally. No distinc- 

 tion between midgut and hindgut. 

 Wall of Pharynx and dorsal wall 

 of midgut consist of single layer 

 of columnar cells, heavily laden 

 with yolk granules. Ventral wall 

 of midgut irregulär yolk cells. 

 Liver pear-shaped evagination 



Mandibular, hyoid, 



first branchial and 



common anläge of 



fourth and fifth 



arches visible 



externally. 



Pronephric 

 ducts coiled, 

 open anteri 



orly. End 



posteriorly 

 about 10" or 

 11" Segment 

 End in mass 

 of mesoderm 

 in close prox- 

 imity to 



entoderm. 



Pericardial cavity 

 large. Heart has 



become a tube. 



Endothelium dis 



tinct. Division into 



auricular and ven 



tricular portion re 



cognizable. Lateral 



blood vessels 



appearing, also 



branchial arches 



and sinus venosus. 



Thickened over 



fore and mid 



brain. 



