RESPIRATORY NET IN ALLANTOIS OF CHICK 409 



data concerning the relations of the capillary net to the ecto- 

 dermal layer of the allantois, which previously belonged to the 

 serosa proper. At the beginning of their development all the 

 vessels are subepithelial; soon, however, a rich capillary net 

 grows and develops inside of the epithelial layer itself, pierces 

 the epithelium and expands freely above the surface of the 

 ectoderm immediately under the egg shell membrane. 



In my previous paper on the equivalence of the hematopoietic 

 anlages,^ I had the opportunity of mentioning the extreme vul- 

 nerability of the vessels of the allantois in later stages. This 

 vulnerabilitj^ and the ensuing hemorrhages are now fully ex- 

 plained by the development of the resphatory capillary net 

 above the ectoderm under the egg shell membrane. 



This peculiar localization of the respiratory capillary net in 

 the outer sheet of the allantois did not entirely escape the ob- 

 servation of Fiillborn. 



Dort wo die Kapillaren des ausseren Blattes ihre typische Ausbildung 

 erlangt haben, finden wir statt des doppelschichtigen kubischen Serosa- 

 Epithels eine Schicht sehr platten Zellen, welche unmittelbar liber den 

 Kapillaren liegen; es erinnert dies ganz an die Verhiiltnisse, welche wir 

 sonst l)ei respirierenden Organen zu finden gewohnt sind. Diese diinne 

 Ektodermlage wird in dem Laufe der Entwicklung von den ilir anlieg- 

 enden Wandungen der Kapillaren immer schwerer zu unterscheiden 

 und in der zweiten Hiilfte der embryonalen Entwicklung gelingt dies 

 meist nicht mehr. 



This statement quoted from Fullborn's paper^ strongly sug- 

 gests an immediate contact between the respiratory capillary 

 net and the egg shell membrane, but according to Fiillborn, it 

 seems to have resulted from a secondary degenerative process to 

 which the ectodermal cells succumb. The stage of intra ecto- 

 dermal locaUzation of the capillary net evidently was not ob- 

 served by Fiillborn. An epithelial-hke layer of cubic cells be- 

 neath the capillaries is, however, mentioned, but they were 

 interpreted as of mesodermic origin. Thus the outer sheet of the 

 allantois in Fullborn's description seems to be deprived of ecto- 



3 Equivalence of different hematopoietic anlages 1. Spleen. The Amer. 

 Jour, of Anatomy, vol. 20, 1916, p. 255. 

 " Loc. cit. 



