682 REPORT— 1894. 



by a gradual spreading of epiblast cells outside the more primitive bypoblastic 

 wall. 



However ingenious these speculations may be, the author holds them to be 

 erroneous. 



As an example of a mammal the early development of which furnishes us 

 with decisive evidence in this respect, the author wishes to call attention to 

 Tupnja javanica, a small insectivorous mammal from the Malay Archipelago, of 

 which a stufled specimen was exhibited. 



Of this mammal the author possesses a most complete series of preparations 

 of the early developmental phases, including the extrusion of the polar bodies, the 

 fecundating process, the segmentation, &c. Selections from these preparations 

 were brought over by him to the meeting, which were demonstrated to those who 

 were desirous to look at them more closely. 



On the plate exhibited to the Section a few of the more important stages were 

 figured by which the growth of the didermic blastocyst is elucidated. 



It was seen that during the early stages of cleavage of the ovum there is no 

 sensible difference in size of the cells then arising. Still, as early as the solid 

 morula-stage, there is an unmistakable outer layer and an inner core of cells, the 

 latter increasing from one to about a dozen cells. When the latter number is 

 exceeded a cavity arises, the outer layer becomes the wall of this early mono- 

 dermic phase, and the inner core is massed together. 



"With equal rapidity, however, a further differentiation of this inner core into 

 a layer of ilattened cells and a knob of more cubic ones is now inaugurated, the 

 former arranging themselves into the inner wall of what then becomes the 

 didermic blastocyst, the latter being at the outset a local thickening of this inner 

 layer. The outer layer forms a closed sac over and above the inner laj'er and the 

 thickened knob. The outer layer is what the author has called the trophoblast, 

 the inner layer the hypoblast, the thickened knob containing the material out of 

 which both the epiblast and the hypoblast that are going to contribute towards 

 the formation of the embryo itself'will be evolved. 



This takes place simultaneously with a rapid extension in size of the didermic 

 blastocyst. 



The embryonic knob may be said to have a more flattened and a more convex 

 surface ; the former is applied against the trophoblast, the latter protrudes into 

 the cavity of the blastocyst. Where these two surfaces meet, the peripheral 

 hypoblast and the embryonic knob are connected together. 



Soon, however, the convex surface of the embryonic knob is seen to be 

 gradually converted into a cell layer, which remains in connection with the 

 peripheral hypoblast, but which gradually becomes separated from the rest of the 

 embryonic knob. 



An expanse of cells has then been interpolated into the primitive hypoblast of 

 the early didermic stages, in the region where the embryo is going to be evolved — 

 i.e., in the region of the embryonic shield. 



As yet the epiblast is, however, not expanded into a shield, but folded together 

 in the embryonic knob. The first indication of its expansion is a dehiscence in 

 the central portion of the knob, by which the hemispherical knob becomes con- 

 verted into a hollow cup. The upper rim of this cup at the same time becomes 

 confluent with the trophoblast that overcaps it, the convexity of the cup becomes 

 lessened, and the trophoblast then no longer covers the embryonic epiblast. 

 Finally, the convexity altogether disappears, the hollow cup surface is stretched, 

 and the flat or slightly curved embryonic shield has come into existence. The 

 hypoblast below the embryonic shield is much less flattened than the peripheral 

 hypoblast. 



This may be called the final stage of the didermic blastocyst. It is directly 

 comparable to the similar stage of other mammals, and somewhat more indirectly 

 to that of the Sauropsida, with a considerable amount of food-yolk. 



The formation of mesoblast, with which we will not here occupy ourselves, is 

 very soon inaugurated. 



The phenomena described above leave no doubt but that the wall of the tran- 



