YOUNG TWIN HUMAN EMBRYOS WITH 17 19 PAIRED SOMITES. 17 



gestive of a precocious formation of the notochordal process of the sclerotome in somites 

 yet to be differentiated, and is perfectly similar in shape and position with the aotochordal 

 processes already formed. 



Embryo V is almost identical with Embryo VI. There are 17 pairs of somites, com- 

 plete and distinct, and an eighteenth pair is to be seen forming in the undifferentiated 

 mesoderm. The aortic processes of the second, third, and fourth somites, and the noto- 

 chordal processes also of the same, have fused into common masses. Back of this, all 

 processes remain separate from those preceding and succeeding. There is thus fusion of 

 one more aortic process, but of three less notochordal processes than in its twin. The aortic 

 processes end in the twelfth segment, there being thus one less than in Embryo VI. 



THE NOTOCHORD. 



The notochord extends continuously from just behind the hypophysis cerebri to the 

 caudal region (plate 1) and is clearly distinguishable throughout the whole course, although 

 varying greatly in its degree of development in different regions. It will be necessary, in 

 order to obtain a proper conception of it, to trace its course and describe its appearance in 

 the various regions. 



The notochord first appears immediately posterior to the hypophysis and is here 

 formed by a heaping up of cells around a median groove on the dorsal wall of the foregut. 

 Bremer, Broman, Van den Broek, and Ingalls all describe it as beginning immediately 

 behind the hypophysis, just as is found here. Peter Thompson places it at the flexure 

 between midbrain and forebrain, and Janosik just under the midbrain. Evidently back 

 of the hypophysis is the normal point. In this embryo this point is the extreme cephalic 

 end of the dorsal wall of the gut (plate 2, fig. 1). The groove disappears by the time the 

 region of the first gill arch is reached (plate 2, fig. 2), but the heaping up of cells persists 

 and is somewhat higher and narrower than anteriorly. Over the region of the second 

 gill cleft the median groove again appears in the roof of the pharynx and the notochord 

 is here seen in its most rudimentary condition in this embryo (plate 2, fig. 3), being formed 

 merely of a flattened double layer of endodermal cells lining the groove. Mrs. Gage ( 1906 1 

 has shown that long after the notochord has separated from the endoderm elsewhere it 

 retains connection with it in this region, both in man and the pig, and Huber (1912) has 

 proved that it is in this region that the pharyngeal bursa is developed, due to the evagina- 

 tion of the wall of the pharynx by the pull of the notochord attached here. Neither of the 

 above described the condition in embryos younger than stages showing complete formation 

 of the rod-like notochord, but it is interesting to note that although the notochord is nut 

 yet separated from the endoderm in any of this anterior region, yet this point, correspond- 

 ing to the region where the pharyngeal bursa later appears, can be determined as the place 

 of the very least development of the notochord, and therefore the place where separation 

 will last occur, so giving the conditions described by Huber and Mrs. Gage. Proceeding 

 backward, the notochord becomes thicker, more heaped up, and gradually assumes a spheri- 

 cal form on section, and forms a round rod (plate 2, fig. 4) merely lying in contact with the 

 endoderm. Over the anterior portion of the yolk sac this rod begins to separate from the 

 endoderm and for a short interval mesoderm passes between the two. and then the two 

 dorsal aortse, which have been rapidly approaching each other, fuse immediately over the 

 gut and directly beneath the notochord (plate 2, fig. 5). Over the posterior portion of the 

 yolk sac the aorta again becomes paired and the notochord immediately sinks to come 



