502 A. A. W. HUBRECHT. 



disagreement that a satisfactory solution of the question of 

 gastrulation and mesoblastogenesis in the Amniota is still a 

 desideratum. More than one of the points formally exclude 

 each other (e. g. 6 and 4). Stil]_, it is impossible to arrange 

 them in two or more groups into which the different investi- 

 gators could be respectively brought together. To prove this 

 it may be sufficient to say — restricting ourselves for the present 

 to researches on mammalian embryology — that point 2 is now 

 answered in the affirmative, as far as its first proposition goes, 

 by Kolliker (18, 19), van Beneden (3), and Fleischmann (8) ; 

 in the negative (i. e. affirmative as far as its second proposition 

 goes) by Heape (9), Bonnet (5), and myself (14). 



Point 3 is affirmatively answered by Carius (7), Keibel (17), 

 and van Beneden. 



Point 4 by Heape and Bonnet, against whom Keibel argues 

 at great length. 



Point 5 is very diversely answered, certain authors, as Strahl, 

 considering the paired lateral plates of mesoblast alongside of 

 the median notochord to be lateral forward outgrowths of the 

 posterior mesoblast in the region of the gastrula ridge, whereas 

 others (amongst whom Rabl indulges in more general and com- 

 parative speculation) consider these plates as lateral wing-like 

 growths of the median streak of tissue. 



On point 6 van Beneden has committed himself to very far- 

 reaching speculations, which are viewed with favour and 

 adopted by Rabl (24), doubted by Bonnet (5, ii) and myself 

 (15), combated by Keibel (17). 



This being a verj' general outline of some of the chief points 

 in contest, I do not think it desirable to enter at any greater 

 length into historical retrespects, or to go into more detail 

 concerning the argumentation of the different authors. Many 

 of the papers contain careful comparisons of the results therein 

 brought forward with those of the other authors ; many are 

 ballasted by polemical remarks directed against the authors of 

 such researches as would appear to lead to divergent con- 

 clusions. 



As my object is in the first place a rearrangement of the 



