EARLY STAGES OF DEVET.OPMENT OF THE MOUSE. 77 



venture at present to express a very definite opinion, except 

 that I am incliued to believe that a via media miglit be 

 found, wliicli should avoid those grossly mechanical explana- 

 tions of the origin of the amnion and allantois, against which 

 this author has rightly protested, on the one hand, and on the 

 other the very serious difficulties in which the Monotremata 

 involve us when we attempt to derive the Mammalia directly 

 from an Amphibian ancestor with a small-yolked and holo- 

 blastic ovum. 



Before I conclude I must express the great obligations I 

 am under to Professor Weldon, to whose kind encouragement 

 and sympathy I owe it that I have been able to undertake 

 this piece of work at all ; to Professor Lankester, who allowed 

 me both time and space in his laboratory here last year ; to 

 Mr. G. C. Bourne and Mr. (now Professor) E. A. Minchin 

 for many hints and suggestions ; and last, but by no means 

 least, to Professor Hubrecht, with whom I spent five weeks 

 last Christmas, for his generosity in giving me free access to 

 his laboratory and his preparations, and for his courtesy in 

 placing so much of his own valuable time at my disposal. 



Oxford, July, 1899. 



Postscript. 



Since the above was written van Beneden has published 

 in the ' Anatomischer Anzeiger ' of last September an account 

 of the development of the bat (Vespertilio murinus), which 

 to my mind completely disposes of Duval's hypothesis of a 

 " stade didermique primitif," and of the origin of the embry- 

 onic epiblast from the trophoblast. 



Van Beneden's description and figures show in the clearest 

 way that the epiblast in the bat, as in other forms, is a part 

 of the inner mass ; and I need here do no more than give a 

 few quotations from his paper, including his remarks on the 

 " blastopore " figured by Duval at the anti-embryonic pole 

 of the blastocyst (Duval, fig. 24). For the details the reader 

 must be referred to the paper itself, 



