392 A HUMAN EMBRYO OP THE PRESOMITE PERIOD. 



The ovum was dissected out from the uterus and placed in 10 per cent for- 

 malin (4 per cent formaldehyde) within an hour after the operation. It was placed 

 upon a bed of cotton and appeared at first to be almost spherical. It flattened out, 

 however, and at the time of cutting was ellipsoid in shape. The specimen was not 

 carefully measured at that time. It was embedded in paraffin and cut into serial 

 sections through its equatorial plane by Dr. B. Harrison Willier, Dr. Mateer's 

 laboratory assistant. In all, 277 sections were saved; 2 passing through the embryo 

 and 6 through the extra-embryonic chorion were lost, making a total of 285 sec- 

 tions. Inasmuch as the microtome was set at 1(V, this, the shortest, diameter of 

 the chorion may therefore be placed at approximately 3 mm. Forty-nine sections 

 through the region containing the embryo were stained at once in carnr'ne. The 

 remaining sections were mounted on slides and, in February 1916, were forwarded 

 to the Carnegie Laboratory of Embryology, where they were stained by various 

 dyes, including hematoxylin and eosin, iron hematoxj'lin, eosin, aurantia, and 

 orange g. 



The 49 sections through the embryo were from the first kept in serial order. 

 The order of the other sections through the extra-embryonic region was only par- 

 tially preserved, but has since been restored as far as possible. This necessitated 

 the renumbering of the entire series of sections, and throughout this paper the new 

 serial numbers will be uniformly used. 



SECTIONS THROUGH EMBRYO AND ADNEXA. 



In order that the reader may trace the various structures making up the 

 embryo and its adnexa, there will be given here a systematic description of the 

 individual sections. The form of the structures is diagrammatically shown in figures 

 1 and 2. While the microtome was set at 10, measurements show that a few of the 

 sections were cut irregularly, some being more, others less than 10/z. Where this 

 occurs note will be made of it. 



The chorion as a whole presents the form of a much flattened sphere, and the 

 sections pass through its equatorial plane. The flattened surfaces may be regarded 

 as the two poles ; the one to which the embryo is attached we may call the placental 

 or dorsal pole, and the opposite is the ventral pole. These poles are dorsal or ventral 

 as regards the original position of the embryo; that is, the dorsal surface of the 

 embryonic shield is towards the dorsal pole of the chorion, and the ventral surface 

 towards the ventral pole. 



