520 A. A. W. HTTBEECHT. 



find them in Maramaliaj with what we notice in the anaraniotic 

 Amphibia, Cyclostomata, and Amphioxus. 



It appears to me that the principle of precocious se- 

 gregation, which plays such an important part in ontogeny, 

 will serve to explain many of the riddles of which even such an 

 acute and eminently painstaking observer as Bonnet complains. 

 In our case this principle must be applied to part of the 

 hypoblast. 



1 assume — and we actually observe the fact in the 

 opossum (27), the mole (9), the hedgehog (15), the shrew, 

 the rabbit (1, 19), the bat (2, 3) — that in the monodermic 

 stage of the blastocyst certain cells forming part of its wall 

 separate from this and arrange themselves into a layer, which 

 either at a very early (hedgehog) or at a later stage (rabbit) 

 forms a closed sac inside of the outer layer.^ A didermic 

 stage of the blastocyst is thus inaugurated before the 

 actual process of gastrulation has set in. It seems 

 to me that we have here an eminent example of precocious 

 segregation, the determining cause of which I will discuss 

 later on (pp. 529—533). 



As I have supposed that only a portion of the hypoblast 

 has partaken in this precocious segregation, another portion of 

 it can be expected to arise in a more palingenetic fashion." 

 This we actually notice in the gastrula ridge, and I have 

 already above (p. 500) referred to the results of Balfour, Rabl, 



^ Van Beneden has pretended that in the rabbit the hypoblast remains 

 absent at the pole of the blastocyst opposite the embryo. Henseu, on the 

 contrary, has figured (1. c, pi. viii, fig. 18) liypoblast-cells at tlie incriminated 

 spot in an early didermic stage. Keibel (1. c, pi. xxiv, fig. 4Ga) again agrees 

 with van Beneden, at least for the early stages. 



2 In the ' Anatomischer Auzeiger,' Band iii, 1888, p. 911, I have already 

 hinted at the probability of the existence of two separate phases in which the 

 process of gastrulation of the Mammalia has become subdivided. Keibel has 

 taken up this suggestion in his essay above referred to (' Archiv fiir Anat. 

 u. Phys.,' 1889, Anat. Abth.), but has not worked it out any further, which 

 is for the first time done in this paper. Kcibel's paper contains a severe but 

 well-founded criticism of van Beneden's theory of the blastophore and 

 lecithophore. 



