528 A. A. W. HUBRECHT. 



raesoblast-cells by kaiyolytic cell- division. This latter point, 

 however, is, as we shall see, to a certain extent secondary, in 

 the same way as we have judged it secondary, whether in 

 Amphibia mesoblast was produced from a larger or from a 

 smaller extent of surface belonging to the hypoblast-cells that 

 will finally constitute the lower and posterior wall of the larval 

 intestine. 



The difficulty that remains is this : is there any possibility 

 of comparing that hemispheral surface belonging to the lower 

 and posterior portion of the larval amphibian hypoblast with 

 the annular zone observed in mammals ? I think this comparison 

 will ofi'er no difficulties if for a moment we were to suppose 

 the larval amphibian when it was in the stage of fig. 92 to 

 increase by the addition of food-yolk. It might then be ex- 

 pected to expand ventrally, the actual cells which were after- 

 wards to partake in the formation of the ventral wall of the 

 gut being pushed aside, whereas at the same time a further 

 inferior expansion of both epiblast and hypoblast furnished a 

 sac in which this increased yolk might be expected to find its 

 place. Of the state of things here described I have given an 

 outline sketch in fig. 93, to the details of which I will presently 

 return. It requires no straining of the imagination to picture 

 to ourselves fig. 93 here alluded to still further expanding into 

 a spherical sac, on the top of which the future embryonic tissue 

 was fiattened out, and we then immediately see that an annular 

 zone of hypoblast would thus make its appearance (fig. 95) under- 

 lying the free borders of the embryonic epiblast, and contribut- 

 ing, when once the folding off of the embryo might have set 

 in, towards the formation of the ventral and posterior wall of 

 the gut. This assumption of a very considerable increase of 

 food-yolk indeed serves to explain the change of shape and 

 size, the origin of a vascular area on the yolk-sac, &c. 



We have reason to expect that between the Amphibia and 

 the Hypotheria a phylogenetic link has once existed in 

 which actual food-yolk formed a very considerable addition to 

 the early blastocyst. The case of the Ornithodelphia is most 

 important in this respect. There is little or no food-yolk in 



