«6 J. W. JENKINSON. 



the irregularly folded surface of the trophoblast (I have 

 shown a section of a similarly distorted blastocyst in ray 

 fig. 6). 



And lastly, in figs. 8 and 9, he has made the distal and 

 lateral trophoblast continuous^ not with the proximal tropho- 

 blast overlying the epiblastic knob (the distinction in staining 

 properties which he has introduced is, in my opinion, much 

 exaggerated, and in any case of insufficient value by itself), 

 while in figs. 10 to 12 he has ignored this layer altogether 

 (compare figs. C and D). With regard to Robinson's 

 account of the isolated stage he had of the ferret, according 

 to which there is here also an epiblastic disc resting on the 

 surface of a hypoblastic vesicle, I think it must be regarded 

 as probable that he was not unbiassed by his previous inter- 

 pretation of the development of the mouse. But whether 

 this description is correct or not, I hardly think it justifiable 

 to base a speculative theory on an individual case, as Robinson 

 has done (and he is not the only one), more particularly in 

 mammalian embryology, a subject by no means easy to work 

 at in itself, and in which it is peculiarly difficult to bring the 

 phenomena into line with what is known of the development 

 of other Vertebrata. Before, then, I proceed to criticise the 

 theories of Robinson, Assheton, and Duval, I may perhaps be 

 allowed to briefly review the history of our knowledge of the 

 embryogeny of the Mammalia. 



As is well known, the first accurate account of the de- 

 velopment of the mammalian ovum was given by van 

 Beneden, who published in 1875 a very careful description 

 of the segmentation and formation of the germinal layers in 

 the rabbit. According to him, one, the larger and clearer of 

 the first two blastomeres, divided more rapidly than the 

 other, and there resulted an embryo composed of an outer 

 layer of clearer cells derived from the former, surrounding, 

 except at one point, an inner mass of somewhat larger and 

 darker cells derived from the latter. The outer layer van 

 Beneden termed from its subsequent fate ectoderm, the inner 

 mass endoderm ; he named the whole a metagastrula, and 



