432 IMPLANTATION OF THE OVUM IN 



accurate concerning the details of that very important but still undescribed stage, 

 the preplacental period of the human embryo. 



In the youngest human embryo yet described Peters ('99) has shown that the 

 blastocyst passes through the uterine epithelium into the connective-tissue stroma; 

 this is unquestionably due to the perforatory action of the trophoblast cells, as has 

 been demonstrated for Cavia and is now shown in Spermophilus. It may not be 

 unreasonable to suppose that in many tubal pregnancies the trophoblast by some 

 precocious development or by reason of delay in the passage of the ovum through 

 the oviduct reaches a condition of maturity whereby it attaches itself to and per- 

 forates the epithelium of the tube in a manner similar to that just described. 



III. BIBLIOGRAPHY. 



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