412 A Three Weeks' Human Embryo 



Comparing this specimen with the His models and Athis," it is seen 

 that tliere are some resemblances to his specimen Lr estimated at twenty- 

 one days, but in external appearance, it seems to lie between a and R 

 estimated by him to be from 21 to 25 days. 



The specimens Xo. 164, 209, 148, and 80 in the Johns Hopkins collec- 

 tion show that in certain features of development they form a progressive 

 series (see ISrephric System, below). The formula devised by Mall,^ for 

 determining the age (days = yJlQO x length of embryo), approximately 

 worked out gives the following : 



TABLE I. 

 No 309 164 148 80 



Length in mm 3+ 3.5 4.3 5 or 4.5 



Myotomes 19 28 to 29 



Days 17+ 19— 21- 22 + 



The Alimextary Canal and its Appendages. 



Mouth. — The mouth (Figs. 1-4) is simply a cleft between the fore- 

 brain region and the mandilile, extending laterally to the just forming 

 maxilla and at its tips having a thin membranous portion (Fig. 5) in 

 section strongly resembling the membranous tips of the gill-pouches. 



No remnant of the oral plate was found. The position of the dorsal 

 limit of the ectodermic portion of the oral plate is indicated by the hypo- 

 physis (Figs. 9, 3, 4). The latter is a bi-lobed, widely opened pouch in 

 contact with the infundibulum of the brain and partially surrounding it. 



Pharynx. — No signs of Sessel's pocket, the most cephalic of the ento- 

 dermic structures of the pharynx could be found since the sections are not 

 favorable in this region. 



The lateral or lingual folds, described by Kallius * as forming the first 

 rudiment of the tongue, are represented in the mesal view (Fig. 4), as a 

 ridge lying at the side of a mesal pit. The iuherculum impar has not yet 

 arisen in front of the median thyroid. The latter body with the pit from 

 which it arises is present, but the tubular connection between gland and 

 pit has nearly disappeared (Figs. 3, 4). 



The floor of the pharynx is partially exposed in Fig. 11, showing lateral 



^ His, W., Anatomie Menschlicher Embryonen. Text and Atlas. Leipzig, 

 1880-1885. 



"Mall, F. P., Age of Human Embryos. Ref. Handb. Med. Sci., 2d Ed.. III. 

 1901. 



•Kallius, E., Anat. Gesell., Verhand, 1903. 



