RECONSTRUCTION OF TWO SETS OF DUCK TWINS, WITH SPE- 

 CIAL REFERENCE TO THE EARLY EMBRYONIC DEVELOP- 

 MENT OF THE VASCULAR SYSTEM 1 



By Edward S. Ruth 

 (From the Department of Anatomy, University of the Philippines) 



TWO PLATES 



The early stages of a large series of chick and duck twin 

 embryos have been studied especially by Kaestner, (2) and a 

 few have been recorded by Burckhart,(i) Reichert,(4) and Mit- 

 chell. (3) However, up to the present time only a few of the 

 embryonic twins have been reconstructed. Kaestner apparently 

 was the pioneer in making reconstructions of young twin em- 

 bryos. Those figured by him are considerably older than the 

 duck twins under discussion in this paper. Much may be eluci- 

 dated regarding the early formation and intimate relation of 

 embryonic twins by careful detailed study of the serial sections 

 and especially by making a reconstruction. In a previous com- 

 munication (5) I described four sets of duck twins. Set III 

 of that paper has been reconstructed and is figured in this con- 

 tribution ; set V was found subsequent to the issue of the pre- 

 vious publication. These two sets of duck twins have been re- 

 constructed according to the Borne method. I wish to express 

 my appreciation and thanks to Dr. Ricardo Molina for the re- 

 construction of duck twins set V. 



DUCK TWINS SET V 



The duck twins are in the 9-somite stage of development. The 

 head and thoracic regions lie parallel to each other, while the 

 caudal extremities diverge, forming an angle of about 55°. Sur- 

 rounding the embryos is the area pellucida, which is transparent. 

 It is more marked on the ventral side of the head in the region 

 of the proamnion. 



Ventral side. — The heads are abnormally large and some- 

 what club-shaped. In the region of the fovea cardiaca the 

 membranes have an increased opacity, extending laterad to the 



' Received for publication October 13, 1917. 



201 



