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



into contact with the endoderm, with which it is connected by a narrow neck of cells 

 (plate 2, fig. 6). It is in this region that the notochord attains its largest diameter. 



In the dorsal wall of the hindgut a median longitudinal groove now develops along 

 the line of attachment of the neck of cells from the notochord and coincidently with the 

 deepening of this groove the notochord loses its spherical condition, becomes elongated 

 dorsoventrally and compressed laterally into a flask shape (plate 2, figs. 7 and 8), and then 

 passes rapidly into a condition similar to that at the anterior end, forming a heap of cells 

 surrounding the groove. In descriptions of embryos in these early stages, I have found no 

 mention of any condition of delayed development such as this. The rod-like form, once 

 attained behind the pharyngeal region, has been retained to the caudal termination. This 

 condition is small in extent, however, and by gradual transition the notochord is again 

 found spherical in section (plate 2, fig. 9) at the level of origin of the allantois and from 

 this point back to its termination it forms a round rod, separated from contact with the 

 endoderm by the second fusion of the two dorsal aortae between it and the gut. It ends 

 abruptly in the tail (plate 2, fig. 11) just beyond the termination of the dorsal aorta, at the 

 same level as the end of the postanal gut, and here lies in contact with the medullary plate 

 and widely separated by mesoderm from the postanal gut. 



The notochord was found by Low (1908) not to extend as far caudad in embryo Pfan- 

 nenstiel III as the gut; it did not pass into the tail, but ended over the cloaca. All other 

 authors unite in stating that it passes into the tail, to fuse there with the tissues of the 

 primitive streak — exactly what has been found in this case. 



From the foregoing description it will be noticed that the stage of least development 

 of the notochord is exhibited over the anterior part of the pharynx, the next stage over the 

 hindgut, while the greatest is over the posterior portion of the yolk sac. The complete 

 separation of the notochord from the endoderm is evident only in two regions, the first 

 over the posterior two-thirds of the yolk sac, the second over the whole of the cloaca and 

 postanal gut. In each case the two dorsal aortae have fused between notochord and 

 alimentary canal. Mesenchyme only passes under the notochord in one place, namely, 

 just in front of the first fusion of the dorsal aortae. The notochord lies in direct contact 

 with the under surface of the nervous system throughout its whole course, with the excep- 

 tion of two very small intervals. The first is at the extreme anterior end, where a small 

 amount of mesenchyme separates it from the floor of the brain. The second is where the 

 cells of the first notochordal process are just beginning to burrow in around the notochord. 

 All other notochordal processes of the sclerotomes come in contact only with the sides of 

 the notochord. 



In the cylindrical portions of the notochord the cells are large, fairly clear, and are 

 arranged as a concentric layer with their nuclei near the periphery. The nuclei are large, 

 round, and vesicular. A thin but definite cuticular membrane surrounds the notochord, 

 except in those parts where it has still the form of an endodermal plate and is not yet 

 cylindrical. Low also found no cuticular membrane in his embryo of 13-14 somites, in 

 which the notochord was altogether in the form of a plate, except at the posterior end, 

 where it was free. 



In the embryos described by Van den Broek (1911) there is a cuticular membrane. 

 These embryos are of 21 and 22 paired somites and the only place where the notochord is 

 not free from the endoderm is over the pharynx. The absence of a membrane in this 

 region is not mentioned. The embryo described by me is midway between these two and, 

 as shown, the membrane is only present in the cylindrical portions. 



