506 A. A. W. HUBEECHT. 



we bear iu mind that in the rabbit (Rauber, a. o.), in the mole 

 (Heape), in the hedgehog (Hubrecht), perhaps also in the bat 

 and in other mammals, either isolated cells (Deckzellen) or a 

 continuous layer of cells are present outside of the layer that 

 is going to become the embryonic epiblast. These outer 

 layer cells in some cases disappear or fuse with the embryonic 

 epiblast (rabbit) ; in others they are separated more fully from 

 it in later developmental stages (hedgehog). 



In the shrew similar cells have been found by me ; they do 

 not, however, form a continuous nor a substantial layer, as in 

 the mole and hedgehog. They are detected in the didermic stage, 

 and figured on PL XXXVII, fig. 26, tr' . Their presence here 

 can hardly leave a doubt that also in the earlier stages the tro- 

 phoblast-cells with their smaller nuclei stretch above the em- 

 bryonic knob. If we keep in mind that the nuclei of the 

 trophoblast are in the beginning very wide apart, we cannot 

 wonder that in sections through the earliest embryonic knob 

 fhe outer trophoblastic covering cannot always be indubitably 

 traced. 



Certain of the sections figured give, however, indications 

 that also in this earlier stage the embryonic knob may be said 

 to be situated (figs. 6, 7, and 10) within the trophoblastic 

 layer, and still to be continuous with it at the border. The 

 latter connection distinctly prevails later on (figs. 28 — 31, 

 &c.), when the sparse '^^ Deckzellen" have wholly disappeared. 



(2) Is the difference in size of certain cells composing the 

 embryonic knob which is noticed in one of the specimens of 

 the earliest phase (although I have not as yet specially men- 

 tioned it) a distinction by which at that early age the mother- 

 cells of the later epiblast- cells and of the later hypoblast-cells 

 are distinguished ? Or is it an individual and fortuitous occur- 

 rence peculiar to one of those early specimens? (fig. 5). 



I cannot definitely answer the question, but will merely 

 call attention to it, remarking at the same time that in the two 

 other specimens of the same age (figs. 6 and 7), as well as in 

 those that follow (figs. 8 — 11), I could not recognise any diflfer- 

 ence in size between the cells composing the embryonic knob ; 



