RESPIRATORY NET IN ALLANTOIS OF CHICK 411 



A peculiar interaction between ectoderm and ingrowing ves- 

 sels can be observed during the earlier stages. The ectoderm 

 cells of the deeper layers in the vicinity of the vessels undergo a 

 rearrangement and form conical projections directed toward the 

 vessels; individual ectodermal cells send out processes toward the 

 vessels, as if they were actively attracted by them. Intimate 

 connections are established between the ectoderm and endothe- 

 lial cells and the two structures would be indistinguishable 

 from one another were it not that the large nuclei of the ecto- 

 dermal cells and the specific staining reaction of their cytoplasm 

 offered criteria adequate to differentiate them from the mesoder- 

 mic elements of the vascular walls. 



In somewhat later stages the intraectodermal position of the 

 respiratory capillary net undergoes a further modification. At 

 the thirteenth to fifteenth dsLj of incubation the capillary net is 

 situated above the ectoderm. The meshes, in earlier stages sur- 

 rounded by epithelial ectodermal cells, now come at least by one 

 of their surfaces into immediate contact with the egg shell mem- 

 brane. The net itself becomes gradually closer, the individual 

 meshes touch each other and in a few places only are separated 

 by small islands of ectodermal tissue. The lumina of the capil- 

 laries are surrounded by a distinct endothelium, the nuclei of" 

 which markedly differ from the nuclei of the ectodermal cells. 

 The capillary net now situated immediately under the egg shell 

 membrane is connected by numerous anastomoses with the larger 

 vessels of the deeper mesodermal layers of the allantois. 



At this stage the ectodermal epithelial membrane is found 

 entirely beneath the capillary net, it is traversed by the numerous 

 anastomoses between larger vessels and the capillary net; which 

 lies on its surface. The ectodermal cells retain the characteris- 

 tic features of the earlier stages. Only a few of them reach the 

 surface of the allantois projecting between the individual meshes 

 of the net and come into contact with the egg shell membrane. 

 The structure of the outer sheet of the allantois offers an ex- 

 ample of unique relation between the derivatives of the germ- 

 layers. The derivative of the mesoderm, the capillary net, 

 actually traverses the ectodermal membrane and expands above 



