132 



loidean region, become enlarged into megalokaryocytes, and tro- 

 phoblastic lacunae are excavated in which maternal blood circu- 

 lates. Extensive changes meanwhile occur in the subepithelial 

 tissue, especially in that which overlies the allantoidean region. 

 Lastly, the original lumen of the uterus disappears and a new 

 lumen is formed on the anti-mesometric side of the embryo. The 

 processes which in the embryo result in the formation of the 

 allantoidean trophoblast, the amnion, the yolk-sac, and the allan- 

 tois have been so often described (see especially Selenka) that it 

 is unnecessary to again enter into these matters here, and we may 

 therefore turn at once to a consideration of the changes that take 

 place in the omphaloidean trophoblast and adjacent maternal tis- 

 sues during this, and also (for the sake of convenience) the fol- 

 lowiug periods. 



As soon as the embryo is established in its pit, the uterine 

 epithelium commences to degenerate. The histolytic changes ac- 

 companying this degeneration have already been described. During 

 the process the trophoblast of the embryo becomes firmly attached 

 to the denuded subepithelial tissue (Fig. 13), from which it is at 

 first sharply marked off by a thin, finely fibrillar membrane. This 

 might at first be taken for the basement membrane of the epi- 

 thelium, but must rather be regarded as a thin layer of inter- 

 cellular substance which previously separated the epithelium from 

 the subjacent cells. The cells of the trophoblast may now be seen 

 to be absorbing the fat secreted by the epithelium (Fig. 13). Up 

 to the present they have retained the flattened shape character- 

 istic of them when the blastocyst was free, and mitotic divisions 

 significant of the rapid growth of this as of the other embryonic 

 layers, continue for some time to be of frequent occurrence. Soon 

 however changes set in preliminary to their transformation into 

 megalokaryocytes. 



The cells first enlarge and begin to penetrate often quite a 

 considerable distance into the subepithelial tissues and may even 

 be found in the maternal blood-vessels. (Fig. 17). 



The cytoplasm does not stain intensely with acid stains, and 



