YOUNG TWIN HUMAN EMBRYOS WITH 17-19 PAIRED SOMITES. 9 



embryo 2 to 3 mm. in length belongs to the fourth week, one 5 to 6 mm. in length to the end 

 of the fifth, so that one 3 to 4 mm. long should belong to the early part of the fifth week. 



In Keibel and Mall's Human Embryology (1912) it is also stated by Mall that the 

 ages of early human embryos have all been underestimated previously by about 10 days, 

 so that adding this amount to the results arrived at by comparing this embryo with His's 

 results, would give almost 5 weeks as its age. The range of variation in the age of this 

 embryo lies thus between 18 days on one hand and 30 to 35 on the other, according to the 

 authority chosen, and it becomes impossible to state which of two coitions is responsible 

 for fertilization, even though they are separated by an interval of 3 weeks and have a men- 

 strual period intervening. 



The length of Embryo V throws no light on this question. Indeed, at first glance, it 

 seems to offer a contradiction. This embryo measures 2.75 mm., according to the serial 

 sections, being 0.60 mm. shorter than its twin. It is of exactly the same stage of develop- 

 ment, however, and the discrepancy in length is accounted for by the presence of a very 

 deep concave dorsal bend, while the corresponding bend in Embryo VI is very shallow, 

 measurements thus including almost the whole extent of the back in the latter but not in 

 the former. Embryo V is almost identical in development with Embryo VI, but is not 

 actually an identical twin, as the two embryos are the products of two separate ova, as 

 shown by the presence of a separate complete sac for each. 



EXTERNAL APPEARANCE. 



In external appearance Embryo VI resembles most closely embryo 6 (figs. Vr and Vv) 

 in Keibel and Elze's Normentafel (1908). This is embryo Pfannenstiel III, described by 

 Low. There is also a close resemblance shown to figure 4 of His's Normentafeln (1880). 

 The head of the embryo is rounded and small, and shows on each side a slight swelling mark- 

 ing the position of the optic vesicles. Between these is the anterior neuropore, still a fairly 

 wide opening. The head is sharply flexed on the long axis of the boch r , forming a well- 

 marked cephalic bend, and lies immediately in front of the very prominent, bulging heart 

 region. Between these two parts of the body lies a wide slit, which narrows rapidly as it 

 passes inwards and forms the primitive mouth cavity or stomodseum. Between it and the 

 pharynx is a distinct quadrangular membrane, the buccopharyngeal membrane, which is 

 still complete, except for 3 small, circular perforations, the membrane evidently just begin- 

 ning to rupture. In embryo Pfannenstiel III, of 14 somites, described by Low (1908), it 

 is still complete, and also in embryo Bulle, of 14 somites, described by Kollmann (1890). In 

 the embryo with 23 somites described by Peter Thompson (1907) it has just torn com- 

 pletely, so that this embryo of 18-19 somites, with perforations just beginning, fits exactly 

 into the series. 



For two-fifths of its entire length the embryo lies spread out wide open over a large 

 yolk sac, and is much flattened dorsoventrally. The yolk sac, at its junction with the body 

 of the embryo, shows no constriction whatever laterally, but only in front and behind, so 

 that the yolk stalk apparently is only beginning to be differentiated. Over the posterior 

 part of the yolk sac there is a shallow but distinct concave dorsal bend in the body of the 

 embryo, and immediately posterior to this the body is rounded and bulging and exhibits 

 a convex sacral bend as it runs rapidly to its termination in a small, blunt, conical tail. The 

 wide yolk stalk, concave bend, and short, blunt, posterior portion of the body are almost 

 identical in appearance with those of the embryo described by Low (1908). Other similari- 

 ties are to be seen in an appreciable interval between the yolk stalk and belly stalk, and in 



