YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 1 1 



with 13-14 somites, 3 gill clefts are visible externally, while in this embryo, with 18-19 

 somites, there are only 2. Only 2 are to be seen externally in Janosik's ( 1887) embryo of 

 3 mm. and Wallin's (1913) of 2.3 mm. Again, in the 2 described by Van den Broek, with 

 22 and 23, and 1 by Thompson with 23 somites, there are 3 clefts, the same number as in 

 Low's embryo, which thus seems to show a precocious formation of the third cleft. For 

 if 3 were normally present at the 14-somite stage, at a stage with 9 somites increase we 

 would expect the appearance of a fourth cleft, which is not found until embryos are reached 

 such as Bremer's (4 mm. in length), Mrs. Gage's (4.3 mm. long with 28 somites), or Ingalls's 

 (4.9 mm. long with 35 somites). 



As in all embryos described in the early stages, this embryo does not show externally 

 the position of the anterior mesodermic somites, but in the region of the concave dorsal 

 bend and the rump they are indicated by noticeable protuberances. 



There is no indication whatever of the limbs. 



The nervous system is still open to the exterior in two places, the anterior and posterior 

 neuropores. The anterior neuropore is a wide opening situated in a shallow depression at 

 the extreme anterior aspect of the flexed head, and lies directly between the optic vesicles. 

 The posterior neuropore begins in the rump, where the roof of the neural canal ends and the 

 canal opens as a deep gutter, whose walls gradually decrease until the gutter form is lost 

 and it forms only a flattened medullary plate extending out on the tail. 



It will be seen, from the above description, that this embryo, while situated exactly 

 half way between two well-known stages of development, resembles embryos younger than 

 itself very much more than older ones, and although much greater in size, in many respects 

 does not seem to be any further developed than the younger ones. The main differences 

 from the above description of Embryo VI, which are shown by Embryo V, are in the pos- 

 session by the latter of a very deep concave bend in the back, a yolk stalk that is definitely 

 constricted at the sides as well as before and behind, and in the fact that the long axis of 

 the embryo practically forms a straight line and not a spiral. The belly stalk passes directly 



backward under the tail. 



THE AMNION. 



There is a well-developed, completely closed amniotic cavity. The amnion itself is 

 formed of two closely applied but quite distinct layers of flattened cells, the nuclei appear- 

 ing at scattered intervals and forming swellings in the cell layer where they lie. The 

 amniotic cavity is fairly extensive and an idea of it can best be given by describing the line 

 of reflection of the amnion from the body of the embryo. The whole of the head and the 

 anterior half of the, heart region are placed entirely within the cavity, the most anterior 

 point of the origin of the amnion being about half way back on the ventral surface of the 

 bulging heart region, in a line convex forward. From here it inclines obliquely upward on 

 each side of the heart, until it crosses the septum transversum, where it immediately 

 changes direction and runs directly caudad on the lateral body wall. The body in this 

 region is so flattened dor so vent rally that only the upper surface is in the amniotic cavity. 

 On reaching the region of the origin of the belly stalk, the line of reflection again crosses 

 the rounded body wall obliquely, this time toward the ventral surface, and passes off on 

 the belly stalk, from the borders of the flattened dorsal surface of which it is reflected right 

 out to the chorion. The whole of that portion of the embryo posterior to the belly stalk 

 thus projects, like the head, into the amniotic cavity, and a narrow, tapering, or funnel- 

 shaped prolongation of the cavity is continued over the upper surface of the belly stalk, its 

 apex being in contact with the chorion. 



