592 SUMMARY OF CURRENT RESEARCHES RELATING TO 



smaller cells. A pseudogastrula is formed. There seems to be " a 

 period of very rapid growth from the beginning of segmentation up to 

 the formation of the pseudogastrula — indeed so rapid that the expansion 

 of the segmentation cavity proceeds more quickly than the rate of 

 absorption of fluid from outside, hence a drawing-in of the less convex 

 pole, i.e. the granular cells, and the production of the pseudogastrula." 

 The invaginated cells become differentiated into two types — a single 

 outer layer of lightly staining cells with definite nuclei and distinct 

 outline, and an inner mass of closely packed elements filled with, 

 granular matter and of ill-defined outline. These cells are evidently 

 concerned with the storing up of food reserves for the free-swimming 

 period, and as this continues the segmentation cavity becomes so much 

 decreased that the further growth must take place outwards, causing the 

 subsequent evagination which leads to the stretching and rupture of the 

 gastral epithelium. 



It is held that the columnar ciliated cells are developed at the animal 

 pole, which is the anterior end of the free-swimming larva, and the pole 

 at which fixation occurs. The origin of the ova, according to the 

 author, is from the choanocytes. This corroborates the view that 

 Porifera are not far removed from colonial Choanoflagellata. 



Influence of Vibrations on Form of Certain Sponge-spicules."' — • 

 Arthur Dendy and J. W. Nicholson have studied the discorhabds of 

 Latrunculia, and more especially the oxydiscorhabd of a related new 

 genus, with a view of determining Avhether the whorls correspond in 

 position to the nodes of a vibrating rod. There is in the oxydiscorhabd 

 a close agreement between the observed and the calculated positions of 

 the whorls. The strength of the currents in the sponge must tend tO' 

 set up vibrations in elastic bodies of suitable shape embedded in the 

 almost liquid mesoglcea. The special accumulation of silica on the 

 nodes would be due not directly to the vibrations of the spicule, but to^ 

 the effect of the vibrations on the formative cells, which may seek out 

 the least disturbed positions. 



Protozoa. 



Eifect of Media of Different Densities on the Shape of Amoebse.f 

 Mary J. Hogue grew an Ammha Umax on agar media of different 

 densities, made up with Witte's pepton. Ou 2*5 p.c. agar the amoeba 

 are rather circular, with few pseudopodia. Compai-ed with those grown, 

 on the thin medium, they are more numerous and often massed together. 

 On 0*5 p.c. agar the amcebge are elongated, with irregular pseudopodia.. 

 They move freely on this medium and are evenly distributed over it. 

 On 1, 1-5, and 2 p.c. agar the amoebge show the forms intermediate 

 between the amoebae grown on 0*5 and on 2 " 5 p.c. agar respectively. 

 The media best adapted to the growth and multiplication of the amoebae- 

 are 1*5 and 2 p.c. agar plus 0*4 p.c. Witte's pepton. 



* Proc. Roy. Soc, Series B.lxxxix. (1917) pp. 573-87 (15 figs.), 

 t Journ. Exper. Zool., xxii. (1917) pp. 565-72 (9 figs.). 



