34 YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 



embryos described by me, although all three embryos are at the same stage reckoned by 

 the number of somites. In an embryo of V. B. length 4.5 mm., N. B. length 5 mm., he 

 finds a short stretch of duct with one tubule on it, opposite the seventh somite, then begin- 

 ning at the eighth somite, the Wolffian duct with 15 mesonephric tubules on the right, 17 

 on the left. Mrs. Gage (1905) found one isolated tubule in the eleventh segment, separate 

 from and just cranial to the beginning of the mesonephros, in embryo 148, three weeks old 

 and possessing 27-28 somites. In two others of similar age she found similar isolated 

 tubules. Tandler (1907), in an embryo with 38 paired somites, found isolated tubules and 

 one glomerulus (presumably external) in front of the mesonephros, but does not state in 

 what segments. Janosik (1887) in a 3 nun. embryo found two isolated pronephric tubules 

 with nephrostomes, and an external glomerulus in front of the mesonephros. Bremer 

 (1906), in a 4 mm. embryo, found cranial to the mesonephros 3 isolated tubules, the first 

 opening into the ccelom, at the level of the liver, by a nephrostome. The mesonephros in 

 all these cases is well developed. Pronephric tubules have thus been found persisting for 

 a long time and always in the region of the seventh to twelfth somites, shown to be that of 

 the greatest development of the pronephros in the two embryos here described and also 

 in those of Felix. If this region degenerated with anything like the rapidity of the parts 

 of the pronephros cranial to the seventh somite (and pronephros is present here also, as 

 shown by the tubules in Dandy's embryo and remains in older ones), these tubules enumer- 

 ated above would disappear long before they do, so that their persistence in this region is 

 rather an interesting fact. 



Regarding the external glomerulus, Felix states that it does not appear until the 

 pronephros is in full process of degeneration. This agrees with its occurrence in Tandler's 

 and Janosik's embryos, and if the statement is true the presence of a glomerulus in Embryo 

 VI and none in Embryo V will explain why the former exhibits only half the number of 

 pronephric tubules found in the latter; Embryo VI therefore shows an advance in the 

 nephric system over that in Embryo V more than commensurate with the possession of 

 only one extra pair of somites, and this advance is further shown by the possession of 

 nearly double the number of mesonephric vesicles that occur in Embryo V. 



Felix (1912) remarks as to the Meyer embryo: "Whether this concentration of six 

 pronephric tubules within the limits of 3| body segments indicates a primary dysmeta- 

 merism or has resulted from the approximation of originally more separated anlagen can 

 not be determined." This suggestion of primary dysmetamerism in the formation of the 

 pronephric tubules is very interesting, as it will be remembered in both Embryos V and VI 

 several of the segments gave rise to 2 tubules each. There are 10 tubules in all in Embryo 

 VI, 18 in Embryo V, of which respectively 4 and 12 are found in pairs (2 opposite 1 

 somite), making a total of 16 out of 28 tubules showing evidence of dysmetamerism. In the 

 mesonephric anlage no definite arrangement of vesicles according to segments is to be found. 



Regarding the genital cells and ridges there is little to say. Between the cloaca and 

 the mesoderm of the body, in a few instances, were masses which might have been either 

 multinucleate cells or very solid clumps formed by a few mesenchyme cells. They resemble 

 the cells known as wandering or primary genital cells, 2 of which, from embryo Pfannen- 

 stiel III, are shown in a figure in Keibel and Mall's Embryology. Whether the cells I have 

 seen are actually genital I am not prepared to state, owing to a certain amount of macera- 

 tion of mesoderm in this region. No genital ridge is yet present. The ccelomic epithelium 

 is 2-layered on the medial side of the urinogenital fold and around the angle from this on 

 to the root of the mesentery. It is thinner in the region of the nephric tissues and 2-layered 



