10 YOUNG TWIN HUMAN EMBRYOS WITH 17 19 PAIRED SOMITES. 



the exoccelom being very large and communicating with the endoccelom by a wide gap 

 extending along the side of the embryo, beginning immediately behind the heart and 

 measuring about half the total length of the embryo. 



It is interesting to note that up to the 14-somite stage most early embryos exhibit the 

 concave dorsal bend, but beyond this is a gap up to the embryo of 23 somites described by 

 Thompson (1907) and two, of 22 and 23 respectively, described by Van den Broek (1911). 

 None of these latter show the bend, but instead a rounded, convex back. The twin em- 

 bryos here described occur in the middle of this interval, with 18-19 somites, and both exhibit 

 this concave bend. The flexure is regarded usually as an artifact and is probably produced 

 as follows : The posterior end of the body is round and solid, and firmly attached to the 

 belly stalk. The anterior end is large, round, and heavy with the immense relative size of 

 brain, pharynx, and heart. The middle connecting part of the body between these two 

 heavy ends is fairly thin and therefore weak, and has the yolk sac suspended ventrally and 

 dragging upon it. In handling the embryo for fixing and embedding, there must be dragging 

 of yolk sac and belly stalk upon the body when the embryo is moved, and this pull in one 

 direction, with the heavy ends sagging in the opposite, practically breaks the back, bending 

 it severely. This appearance is of value, then, only as an indication of the strength of the 

 middle region of the back, disappearing when this part of the body has become sufficiently 

 strong, and because of the relative diminution in size of the umbilical opening and the 

 passage of the amnion out around the whole umbilical cord, binding belly stalk and yolk 

 stalk together and thus strengthening the middle region of the body. 



Coming from the ventral surface of the body, just behind the yolk sac, is the large 

 belly stalk, flattened on its dorsal surface but rounded on the others. It has a course of 1 

 millimeter from embryo to chorion, being thus about one-third the length of the embryo. 

 The embryo lies somewhat twisted upon its long axis, the head to the right over the heart, 

 the tail to the left over the belly stalk. The side to which the tajl turns in the spiral seems 

 to be variable, being to the left here and in embryos described by Wallin (1913), Low (1908), 

 and Mrs. Gage (1905), but to the right in cases recorded by Bremer (1906), Van den Broek 

 (1911), and Ingalls (1907). Mall (1891) describes a case going to the left and says that the 

 bend to the right is more usual. 



The heart forms a very large, prominent bulging occupying the whole ventral body 

 wall between yolk sac and stomoda?um. It agrees with that of the Peter Thompson and 

 Van den Broek embryos in that it projects much farther forward on the right side than on 

 the left. It is interesting to note that in the Ingalls (1907) embryo, 4.9 mm. in length, 

 the heart lies mostly to the left. The position of the chambers of the heart is indicated 

 externally by shallow grooves on the surface, bounding gently rounded prominences over- 

 lying the chambers. 



Starting at the sides of the stomodseum, and passing on each side caudally, is a well- 

 marked groove, or sulcus, which marks off the heart region ventrally from the rest of the 

 body lying dorsal to it. This sulcus extends back to meet the line of reflection of the 

 amnion, where it is lost. Just caudad, and also somewhat dorsal to the stomodseum, is a 

 shallow groove, or depression, running dorsoventrally, its lower end meeting the sulcus 

 bounding the heart. This marks the position of the first gill cleft. A short distance 

 directly caudad, and lying parallel to the first, is the depression of the second gill cleft, 

 about equal in extent but shallower than the first. It also reaches ventrally the limiting 

 sulcus of the heart. Immediately dorsal to the second gill cleft lies the otocyst, a deep, 

 oval depression of the ectoderm, still opening widely to the outside. In Low's embryo, 



