42G SUMMAltY OF CUURKNT HESKAUCHES RELATING TO 



horny sccretiou to escape, which thus assumes the form of a thread; 

 when the egg reaches this portion of the ghmd tlic horny threads 

 become attached to its anterior end ; the same process is continued 

 when the ovum has jtassed through the ghxud. Tiiat the anterior and 

 posterior lihiments of the egg-sliell are really formed in this way was 

 contirnied by direct experiment. 



Cell-Continuity.* — Prof. H. Fol finds that red blood-corpuscles are 

 the (mly elements which are quite isolated. Everywhere else there is 

 continuity between the prolongations of neighbouring cells, and this 

 is true even of cartilage, the elements of which have been regarded 

 as being inclosed each in a continuous membrane. Of all the fixed 

 tissues the nervous system is perhaps the one of which the cellular 

 elements present the least numerous anastomoses. 



Influence of Gravity on Cell-division.f— Dr. 0. Ilertwig comes 

 to conclusions cxa(!tly opposite to those of Pfliiger and Kauber ; he 

 thinks that, of itself, gravity has no direct influence on the division 

 of cells ; the first plane of division, in the eijual eggs of Echinids, 

 may have the most various relations to a perpendicular axis ; and as 

 Roux has shown, frogs' eggs are not hindered in their development if 

 the localized action of gravity is compensated for by the slow revolu- 

 tion of the egg, 



"When the substance of the cell is regularly distributed the 

 nucleus seeks to occupy a central positicm, but if the egg consists of 

 a part which is richer in yolk, and another i)art which is richer in 

 protoplasm, the nucleus alters its position, and passes into the latter 

 part. We see then that the direction and position of the plane of 

 division is primarily dependent on the organization of the cell itself; 

 the nucleus is the direct cause. But the position of the axis of the 

 nucLnis is dej^endeut on the form and differentiation of the jirotoplasm 

 which surrounds it. In many cases gravity has an indirect influence on 

 the orientation of the planes of cleavage, when there are in the cells 

 substances of various specific gravities, which have become arranged 

 in a heavier and a lighter layer. This is often a consequence of the 

 formation of the polar globules and of fertilization ; the egg then 

 gets a definite axis with an animal and a vegetable jiole, and this axis 

 must be perpendicular, on account of the action of gravity ; here the 

 first plane of division must necessarily be perpendicular. 



Origin and Development of Animal Tissues.^ — Prof. E. Hackel 

 sums u]) the results of his studies in the following theses : — 



In all vertebrates the oldest primitive organ is a simple eiiithclium 

 or blastoderm ; this is absolutely preserved in Ampldoxus only, for in 

 the blastula of craniate vertebrates it is more or less cenogenetically 

 modified. From this primitive organ of the first order two primitive 

 organs of the second order arise by invagination — the two blasto- 

 phylls or primary germinal layers of the Gastrnla ; these, again, are 

 palingeuetically preserved only in AmiyMoxus ; in the Cyclostoraata, 



* Arch. Sci. Phys. et Nat., xiii. (188")) pp. 2.56-7< 



t Jenaisch. Zeitschr. f. Naturwit^s., xviii. (1885) pp. 175-205 (1 pi.). 



: Ibid., pp. 20a-75. 



