254 SUMMARY OP OUREENT RESEARCHES RELATING TO 



If we review the facts known about the formation of the meso- 

 derm in the lower Metazoa we see that the two-cell theory does not 

 apply ; for the higher Metazoa, where it does apply, we must suppose 

 that the double-cell-rudiment is the expression of an early differentia- 

 tion, but the mode of formation by the wandering of amoeboid cells is 

 rather a primitive character. This theory agrees with the part 

 taken by these cells, which most constantly appear as phagocytes, 

 or, in other words, retain the function which they have so prominently 

 in the lowest Metazoa — the sponges. 



Influence of Gravity on the Division of Cells.* — Prof. Hertwig 

 has selected the ova of Echinoids for study, as they exhibit equal 

 cleavage and consist almost exclusively of protoplasm ; his experi- 

 ments led him to conclude that gravity exerts no direct influence on 

 the position of the plane of cleavage of animal ova, for in some that 

 were in no way affected the first plane was vertical, in others hori- 

 zontal, and in others again oblique. He objects to Pfluger's conclu- 

 sions, inasmuch as in them and the reasoning therefrom the position 

 of the fertilized nucleus was not taken into consideration. This 

 depends on the external form of the egg-material, and on the way in 

 which the formative and nutrient yolks are distributed in the cell. 

 When the cell-substance is homogeneous the nucleus occupies a central 

 position in the egg, but when one is richer in protoplasm and another 

 in yolk the nucleus tends to pass into the former. The relation of the 

 nucleus to the protoplasm is such that the former always tends to 

 occupy the centre of the active sphere. This leads us to the law 

 which governs the course of the first plane of cleavage ; the direction 

 of the plane is dependent on the position of the axis of the nucleus 

 which sets up division. The position of this nuclear axis is, again, 

 dependent on the form and differentiation of the protoplasm which 

 surrounds the nucleus ; in a sphere the axis tends to lie in the direc- 

 tion of any radius, but in an elliptical body in its longest diameter. 

 When the protoplasmic disc is circular the axis of the nucleus lies 

 parallel to the surface along any diameter. 



Where the constituents of an ovum are of different specific 

 gravities gravity will, of course, have an influence ; but that which it 

 has on the planes of cleavage is an indirect one. 



Perignathic Girdle of Echinoidea.f — Prof. P. M. Duncan describes 

 minutely the anatomical structures in the families Cidaridse, Temno- 

 joleuridse, Echinidse, Echinometridae, and Diadematidee, and the sub- 

 order Clypeastridse, to which the muscles, acting on the " jaws," are 

 attached ; these structures are termed by him " perignathic girdles." 

 In two plates he gives numerous figures of the arrangements in 

 various genera belonging to the above families. 



Apical Area of some Cretaceous and Tertiary Echinids.| — 

 M. Munier-Chalmas has studied the arrangement of the aquiferous 



* Jenaisch. Zeitschr. f. Naturwiss., xix., Suppl. (1885) pp. 70-2. 

 t Journ. Linn. Soc. Loncl., xix. (1885) pp. 179-209 (2 pis.). 

 X Comptes Rendus, ci. (1885) pp. 1074-7. 



