EARLY STAGES OF DEVELOPMENT OF THE MOUSE. 73 



fig. 9, for example, are just in the opposite direction to wliat 

 one would have expected on his theory ; when, moreover, an 

 allied animal, the pig, affords no confirmation of such a view, 

 and the evidence furnished by all other Mammalia is dis- 

 tinctly opposed to it ; and is it not a little unfortunate in this 

 connection that the same author's figures of the rabbit (1) 

 should show a staining reaction which suggests the very 

 opposite interpretation to that which he has advocated here 

 for the sheep (see his figs. 22, 24, 27). If this is so, 

 then the basis of fact upon which Assheton has relied 

 becomes very slender indeed. We will turn, therefore, to a 

 consideration of the general arguments advanced in favour 

 of the hypothesis; and we will admit for the moment that 

 the Mammalia are immediately descended from ancestors 

 with a megalecithal egg, from which the yolk has in all the 

 Placentalia practically disappeared. Assheton supposes (see 

 his diagrams A — D) simply that the epiblast, which in a 

 Sauropsidan grows over the surface of the yolk, has ceased 

 to do so in all mammals except Monotremata, and has instead 

 become included in it; and that the blastocystic cavity cor- 

 responds to the archenterou, which he (and also Robinson) 

 believes to arise in the Sauropsida as a split amongst hypo- 

 blast cells. 



I am aware that the current view of the origin of the hypo- 

 blast, as set forth in the ordinary text-book of embryology, 

 is that it arises from those yolk-nuclei which are separated 

 from the blastoderm by the first tangential division. Now I 

 do not wish to express a positive opinion in this matter, be- 

 cause I have not yet had the opportunity of looking at pre- 

 parations of the blastoderm of the chick during the early 

 stages of segmentation ; but, judging from what I have seen 

 of some very excellent preparations of the early stages of 

 incubation, I should like to suggest that the " Dottersyn- 

 cytium," as it seems undoubtedly to be in Teleostei, and 

 possibly in Elasmobrauchii, and as Mehnert has stated it to 

 be in Emys (22), is also in the chick a structure entirely 

 sui generis, separated at an early stage from the primitive 



