YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 11) 



I find confirmation, in this embryo, of the lumen found in the notoehord by His (1880) 

 in embryo L u 2.4 mm. long, and demonstrated by Eternod (1899) in three embryos of 

 lengths 1.3 mm., 2.11 mm., and the third slightly larger but length unknown. In several 

 places (plate 2, figs. 1, 4, and 9) there are distinct spaces in the very center of the cylindrical 

 portions of the notoehord. They are not very large, and do not extend over more than :i 

 few consecutive sections in any region, but they are perfectly distinct and appear like 

 isolated parts of a lumen not yet completely occluded. In other regions the cells extend 

 right in to the center, occluding the lumen, and frequently one or two extra cells are found 

 in the center of the notoehord. No recent investigators seem to have found evidence of 

 this lumen except Grosser (1913), in the case of a very young embryo where the canal is 

 very distinct. Some authors definitely state, indeed, that it is entirely absent, even in 

 embryos not nearly as large as this one; such, for instance, being the case in the embryo of 

 only 8 paired somites described by Dandy (1910). 



At its posterior free rounded extremity (plate 2, fig. 11) the notoehord is not of exactly 

 the same structure as farther forward. Over the cloaca the radial arrangement of cells 

 with a single peripheral layer of nuclei is lost. The cells become much more crowded and 

 are irregular in disposition, with nuclei lying everywhere closely packed. This resembles 

 very much the illustration of the posterior end of the notoehord given by Low (1908). It 

 ends by fusing with the tissues of the primitive streak, 2 sections later than the fusion of 

 the postanal gut and 3 sections previous to the fusion of the medullary groove with the 

 primitive streak. It here lies immediately under the medullary groove, but quite inde- 

 pendent of it. This posterior portion has been described as ending in three different ways: 

 First, by Low, it is said to end freely midway between medullary tube and cloaca, embedded 

 in the mesoderm. Second, as exemplified by Ingalls and by Thompson, it runs in contact 

 with the under surface of the medullary tube, but fuses posteriorly in the tail with the solid 

 mass of mesoderm of the primitive streak. Third, as described by Bremer and Janosik, it 

 fuses in the tail with the under surface of the nervous system, and this in turn with the 

 primitive streak. The ending described by Low seems to be true of embryos up to the 13- 

 somite stage, as two of 7-8 somites confirm this, one described by Dandy and one by Eter- 

 nod (2.11 mm.), each having the notoehord, as shown in the illustrations, not extending so 

 far caudad as the gut. The second plan described is the normal one, as it is confirmed by 

 the great majority of those who have described young embryos. Fusion with the nervous 

 system into a common mass is really a fusion of each of these structures at a common level 

 with the primitive streak. 



Eternod (1899) is practically the only author who mentions the presence of a neuren- 

 teric canal in embryos of 2 mm. or more. In many descriptions of early embryos it is not 

 mentioned, and it may therefore be inferred that it was not present, and Kollmann ( 1890) 

 remarks positively, in an article on the notoehord, that it had entirely disappeared in an 

 embryo of 14 paired somites. Eternod found it well developed in an embryo of 1.3 mm., 

 another of 2.11 mm., and says that in a third (larger again, but measurements omitted) 

 appearances were practically the same as in the 2.11 mm. embryo. This last embryo had 

 2 gill clefts developed. Reckoning thus, Embryo VI, here described, is of the same stage of 

 development. It is possessed of 18-19 segments, a distinct advance over the one mentioned 

 by Kollmann, and yet it shows distinctly the neurenteric canal. This is, as far as I can 

 ascertain, the oldest embryo in which this canal has yet been demonstrated. The location 

 of this canal (plate 2, fig. 10) is in 4 sections immediately succeeding the caudal edge of the 

 cloaca! membrane, and it is represented by a solid rod-like connection between the upper 



