DESCRIPTION OF A 4-MM. HUMAN EMBRYO. 
J. L. BREMER. 
WitTH 16 TExt FIGURES. 
This embryo, series No. 714 of the Harvard Embryological Collection, 
noted as about three weeks old, is an excellent subject for study because 
of its good preservation and successful sectioning. Unfortunately the 
drawings of the whole embryo are inadequate, so that the sketch given here 
has been altered slightly to conform more accurately with the shape as 
we find it in the serial sections. The necessity for a part of this altera- 
tion may, of course, be due to shrinkage, but the form, as given in Fig. 
7, is certainly at least approximately correct. This shows a large head, 
flexed sharply on the body, a curving back ending in a curled tail, twisted 
spirally to the right; a marked protuberance below the head for the 
heart, and an outgrowth for the fore limb. There is no trace of posterior 
limb. Appended to the ventral surface below the heart is the yolk-sac, 
represented here as cut irregularly, and at the right side of this, 
posteriorly, the body-stalk, cut near the chorion. Four pocket-like de- 
pressions, the gill clefts, lie behind a larger depression, the mouth. 
There is no surface marking to indicate the eye. Protuberances corres- 
ponding to the primitive segments are not shown in the original drawings, 
though plainly visible in a model of the rump region (Figs. 15, 16)— 
perhaps the irregularities of surface have increased, with shrinkage. 
There is no sign of distortion or injury. 
The embryo was preserved in 10 per cent formalin, imbedded in 
paraffin, and cut in a transverse plane. On microscopic examination 
the tissues are found to be excellently preserved; even the frequent 
mitotic figures in multiplying cells show clearly. In fact, by the histo- 
logical condition, as well as by the external appearances, we are led to 
believe that the specimen is normal, and yet the state of growth of some 
parts and the form of others do not agree with the descriptions of human 
embryos of the same size given by His and other investigators. 
The most notable difference is in the stage of growth of the nervous 
system. His describes the thickened medullary plates as having already, 
in an embryo of this size and general stage of development, been changed 
throughout their entire extent from a groove to a closed tube; but in 
AMERICAN JOURNAL OF ANATOMY.—VOL. V. 
