DEPARTMENT OF EMBRYOLOGY. 95 



that this twin is not altogether normal and that, if the pregnancy had 

 not been prematurely terminated, the larger embryo would have gone 

 on to maturity while the smaller one would have remained stationary 

 in size as a stunted cyst, and so would have been entirely overlooked. 

 Careful search at the placental attachment of the umbilical cord might 

 frequently reveal the presence of similar minute epithelial vesicles rep- 

 resenting stunted twins, and thus it is possible that the tendency 

 towards twinning in man is even greater than is now supposed. 



An embryo a little older than the above (No. 1878, Carnegie Collec- 

 tion), in which the second and third pairs of somites are just forming, 

 has been described by Dr. N. W. Ingalls. It represents a stage of de- 

 velopment which has not heretofore appeared in the literatm-e, and thus 

 Dr. Ingalls's paper will be eagerly read by embryologists. Perhaps 

 the most notable feature of the external configuration of the embryo is 

 the elevation of the head region, due jointly to the early growth of the 

 heart and the development of the forebrain. This results in a dorsal 

 flexure of the body. There has been much controversy as to whether 

 this flexure, as observed in slightly older embryos, is normal or not. 

 Judging from the form of this specimen, one would expect to find some 

 degree of flexure as the normal occurrence. 



The foregut is large and well defined. It presents a distinct bucco- 

 pharyngeal membrane, the primordium of the thyroid, and the earliest 

 indication of the first pharyngeal pockets. The midgut and hind-gut 

 are still more or less combined. The cloacal membrane is still in 

 the roof; thus one can not speak of a real hindgut. The allantois 

 is very large, and Dr. Ingalls calls attention to an interesting area of 

 contact existing between its own epithelium and the mesotheUum of 

 the body stalk. The body stalk is very large, being more bulky than 

 the entire embryonic body. Its shape and size are due to the enormous 

 vessels which it contains. There is a distinct amniotic duct. 



By far the most interesting and important results of the study of 

 this specimen are those which pertain to early phases of vascular de- 

 velopment. It is possible to recognize four regions (chorion, body 

 stalk, yolk sac, and embryo) in which blood-vessel formation occurs 

 almost entirely independently, and in each of which the primitive ves- 

 sels show different characteristics. The vascular channels show their 

 greatest development in the body stalk, where the two umbilical ar- 

 teries unite with the venous elements in forming a plexus of enormous, 

 sac-like, anastomosing channels connected on the one hand with the 

 chorionic plexus by irregular, slender, communicating branches, 

 and on the other hand with the right vitelline plexus of the embryo 

 and yolk sac by a slender channel. On the left side there is as yet no 

 communication with the vessels of the embryo. The vessels of the 

 body stalk contain many small, scattered masses of blood cells in vary- 

 ing stages of differentiation. Compared with the body stalk, the ves- 



