CHRONOLOGICAL SUMMARY OF ORGAN ANLAGEN 259 



Stage 23: 9 mm. total body length. 



Mouth developing with embryonic teeth, opercular fold begins 

 backward growth, tongue anlage appears, carotid glands develop 

 from ventral wall of first branchial pouch, epithelioid bodies from 

 wall of second and third branchial pouches, oesophageal plug disap- 

 pears, pancreas anlage is formed, pharyngeal arteries as well as sub- 

 clavians grow from dorsal aorta, mesenteric and lumbar arteries ap- 

 pear, parachordal plate develops in floor of cranium, brain develops 

 anterior choroid plexus, optic lobes, and cerebellum, cranial nerves 

 (except abducens) well developed including all parts of trigeminal. 



Stage 24: 10 mm. total body length. 



Atrophy of mucous glands, mouth with horny jaws and cornified 

 lips takes in vegetable diet, intestine long and looped, operculum 

 closed on right side, thyroid bifurcates and separates from pharynx, 

 spleen anlage appears, two or three branchial clefts break through, 

 mesonephros developing, utricle separated from saccule by oblique 

 partition, all semi-circular canals indicated, optic lobes formed, me- 

 dian cardinal vein appears, and posterior vena cava begins to develop. 



Stage 25: 11 mm. total body length. 



Cilia lost except on tail, operculum completed, cornification of 

 horny rasping papillae of jaws and lips, oesophagus differentiated 

 from stomach, lung buds invested with mesenchyme, pronephros 

 large and conspicuous, retina develops rods and cones, olfactory ap- 

 paratus develops lateral appendix, internal choanae open, olfactory 

 nerve is derived from olfactory lobe neuroblasts, and abducens is 

 formed from neuroblasts of medulla. 



12 mm. total body length stage: 



Olfactory tube develops complete lumen from external nares to 

 internal choanae, thymus separates from dorsal part of first branchial 

 pouch and comes to lie posterior to tympanic membrane near mandib- 

 ular muscles, maximum development of the pronephros which de- 

 generates by 20 mm. stage, cortical (interrenal) adrenal arises from 

 peritoneum in vicinity of cardinal veins, paired gonad primordia 

 prominent. 



