542 Scientific Proceedings, Royal Dublin Society. 



tically corresponds to the ovum of the bat figured above, fig. 1. 

 In fig. 4 the yolk blastoderm has grown over the embryonic area, 

 forming the covering cells (Deckenzellen). Lastly, in fig. 5, the in- 

 vagination of the embryonic area is rectified, and there is a diplo- 

 blastic ovum, the covering cells (Deckenzellen) forming the spurious 

 third layer which misled van Beneden. 



If the above interpretation be correct, it further follows that we 

 have a very abbreviated segmentation occurring in the mammalian 

 ovum. Accepting van Beneden's very precise statements, the inner 

 mass (germinal area) is derived from one of the two primary seg- 

 mentation spheres, the blastodermic vesicle being derived from the 

 other. In other words, the first cleavage furrow demarcates the 

 embryo from the yolk-sac. 



Embryologists are now paying considerable attention to the 

 orientation of the primitive segmentation spheres. 



Agassiz and Whitman (1) have quite recently given a very care- 

 ful account of the segmentation of some pelagic eggs of certain 

 Teleostei (Otenolabrus, Pseudorhomhus melanogaster, P. oblongus, 

 and Tantoga), and they find that the first cleavage plane corre- 

 sponds with the long axis of the future embryo. It is a well- 

 known fact that the nuclear spindles of one cell-generation tend 

 to arrange themselves at right angles to those of the preceding 

 generation. The primary spindle (archiamphiaster), during divi- 

 sion to form a polar globule, almost invariably coincides with the 

 axis of the ovum {i. e. is radial) ; hence the first cleavage spindle 

 usually assumes a position perpendicular to this axis [i.e. is bilate- 

 ral), and the corresponding cleavage falls in a meridian plane 

 which coincides with the median vertical plane of the embryo. . . . 

 " We would carry the generalization one step farther, and say it is 

 highly probable that tlie first cleavage spindle invariably lies at right 

 angles to the axis of the ovum throughout the metazoa, and that there- 

 fore the first cleavage plane is alioays a meridian plane, at least in all 

 oases where the first cleavage spindle is accompanied by cleavage " 

 (p. 33). 



Concerning the mammalian ovum, they say : " The researches of 

 van Beneden on the maturation, fecundation, and cleavage of the 

 mammalian ovum, of which we have thus far received only a preli- 

 minary account, leave it doubtful whether the first cleavage spindle 

 is parallel with the axis of the ovum or at right angles to it. He 

 calls attention to the pronounced polarity of the ovum, but expressly 



