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to Professor Pekelharing of Utrecht for the trouble he took to 

 look over my preparations and discuss my work with me. But I 

 am especially bound to express ray gratitude to Professor Hubrecht; 

 I have now paid three visits to Utrecht, and he has on each 

 occasion most courteously placed at my disposal the whole of his 

 preparations, and a very great deal of his own time as well. It 

 goes without saying that, without his work on the placenta, my 

 own could never have been begun; and perhaps I may be allowed 

 to add that, but for his sympathy and encouragement, it would 

 possibly nevev have come to a conclusion. 



PART I. 



Following Duval's example I divide the processes of placentation 

 into the following four periods. 



Preliminary period up to the fixation of the embryo; this takes 

 place about the sixth day. 



First period from the fixation of the embryo to the formation 

 and attachment of the allantois on about the eleventh day. 



Sêcond period during the penetration of the allantois into the 

 trophoblast; from the twelth to the fifteenth day. 



Tkird period comprising the further changes which take place 

 in maternal and embryonic tissues till the time of parturition ou 

 the twenty-first day. 



For the sake of convenience I shall, after the preliminary period, 

 treat separately of the changes that take place in the omphaloi- 

 dean trophoblast (that adjacent to the yolk-sac) and underlying 

 maternal tissues, and the allantoidean or placental trophoblast 

 and the maternal tissues which lie against it and which it ulti- 

 mately replaces. 



Preliminary period. 



In the virgin or non-pregnant uterus the lumen is fiattened from 

 side to side, in transverse section slit-shaped as a rule but sometimes 

 irregular in outline; into the lumen open the mouths of numerous 

 glands. 



