STUDIES IN MAMMALIAN EMBRYOLOGY. 589 



results to which Bonnet, Heape and myself have arrived in 

 respect to this question had not been obtained, but that in all 

 mammals it was only the protochordal wedge from which the 

 notochord developed, as Carius, v. Beneden and others will 

 have it for the guinea-pig, rabbit, bat, a. o. ; even then the 

 hypothesis that has here been developed would to me seem 

 more acceptable than van Beneden's hypothesis of the blasto- 

 phore and lecithophore. It would at any rate not have to 

 undergo any important modification. 



Should future researches bring to light that indeed the 

 formation of notochord and mesoblast in different mammals 

 takes place according to divergent modes, as would appear to 

 follow from the researches here cited, even then the theory of 

 mammalian gastrulation here developed would remain appli- 

 cable, whereas that of van Beneden collapses as soon as a 

 definite participation of cenogeuetic hypoblast (his lecithophore) 

 in the formation of these tissues has been demonstrated. And 

 this is now the case at any rate for the sheep and for the 

 shrew. 



As, moreover, van Beneden has to deny direct homology 

 between the mammalian inner layer and the hypoblast of 

 Amphioxus, whereas in my hypothesis these two remain per- 

 fectly comparable and homologous germinal layers, I think 

 the latter hypothesis will also have a priori grounds in its 

 favour. At all events, the supporters of van Beneden's view 

 will have to bring Bonnet's and my own results in accordance 

 with their own hypothetical solution. I myself do not see my 

 way to effect this. 



Chap. III. — Points of Comparison in Earlier Investi- 

 gations BY OTHER Authors. 



I have not in the preceding pages very fully and repeatedly 

 referred to the literature of the subject. Whenever references 

 were indispensable they have been inserted. Still a large 

 amount of embryological research which runs along parallel 



