420 A Three Weeks' Human Embryo 



a true pronephrie remnant, or whether a caudal shifting of tlie Wolffian 

 ridges* has taken phice. or whether there is an increase in the nnmber of 

 myotomes in the occipito-cervical region of No. 148 cannot be determined. 

 But at present, it is nnniistakable that in three embryos of 2iA to 3 weeks 

 in development (Nos. 164, 148, and 80 of the Mall collection) there 

 exists a strnctnre which has many resemblances to a pronephros. 



In a 3 mm. hnman embryo, Janosik " found that there is a distinct 

 glomerulus protruding into the coelomic cavity. The position of this 

 glomerulus is similar to the tubules found in the embryos of the Mall 

 collection. 



Tandler " found one glomerulus in eight cases. It seems, therefore, 

 that in spite of the apparent discrepancy of position, there should be no 

 hesitation in expressing the homology of the structure under considera- 

 tion with the pronephros of lower forms. 



Mesonephros. — A preliminary report on the generalized constitution 

 of the mesonephros of Embryo No. 148 was presented with models at the 

 December, 1903, meeting of the American Anatomists.'" The main facts 

 noted^'were, that in the W^olffian ridges an open pronephrie tubule occurs 

 on ^'ach side followed by two groups of mesonephric tubules, the first 

 group of eight being of the usual embryonic type, S-shaped, with Bow- 

 man^s capsule, rudimentary glomerulus, and union with the Wolffian 

 duct; the second group of eleven or twelve, consisting of tubules, none 

 of them uniting with the Wolffian duct and varying from solid aggrega- 

 tions of cells to hollow vesicles and finally to vesicles open to the coelom. 

 The two sides present the same general arrangement although the tubules 

 do not form symmetrical pairs in precisely the same stage of development 

 (cf. Figs. 17-19). 



McMurrich " says, page 391, " One of the characteristics of the mam- 

 malian mesonephros is that it possesses no nephrostomes." Minot ^ in 

 his Embryology, page 237, says, " In all amniota the nephrostomes all 

 become completely separated from both the myotomes and peritoneum 

 throughout the region of the Wolffian body, except that possibly in a few 

 anterior segments, the connection with the peritoneum is retained as is 

 suggested by Sedgwick's observations (in birds)." 



In an embryo 4.25 mm. in length, Meyer ^ showed that in the cephalic 



"Janosik, J., Arch. f. mikr. Anat. XXX, 1887. 



'"Gage, Susanna P., Amer. Jour. Anat.. Ill, 1904, p. VI. 



==1 McMLurrich, J. P., The Development of the Human Body. Phila., 1902. 



^ Minot, C. S., Human Embryology. New York, 1892. 



=^ Meyer, H., Arch. f. mikr. Anat., XXXVI, 1890. 



